Failing to plan shopping days around the times that various shops close for lunch caught us out a few times early on.
Not so much a U.K. > France thing, more a London > Middle Of Nowhere thing, but having to drive everywhere took some getting used to. Even our local store is 6-7 minutes away in my trusty Duster.
Townie!
I wasn’t complaining about it, it was something we had to get used to. If I go into the doctor’s or dentist’s now in the UK I make a point of saying Good Morning. I don’t, however, say goodbye as they do in France.
Interesting you use a duster, I prefer to go on my broom.
Former townie, thank you very much. 
Je vois ce que tu as fait là…
I could fill a page with our ‘struggles’ most of them already been said.
I would say socialising was initially my biggest hurdle, bit of a shy Mary and a non French speaking Mary even worse 
My only stipulation when buying our house was to have only French neighbours, otherwise we would never learn the language. The first year was difficult, but gradually gained the confidence to say more than the basic pleasantries. SF have helped me immensely in recent years 
Even with all the struggles we had we have never once regretted moving to France. We have wonderful neighbours, I would never say I speak French, but it has improved. After many years we now have a English speaking MT, happy days, at our age we cannot afford to lose anything in translation.
Now I am waffling, sorry Cat 
I once had a lovely conversation with a employee in a DIY store, had everything typed out that I needed on Google translate on my phone, showed it to the guy behind the counter who asked me very loudly If I could not have printed the list out in French and given it to him, as I was typing my reply he turned and said to his colleague " yet another thick Britisher who cannot be bothered to learn our language when coming to live in our country", I looked at him and typed out another message to him.
He then said " looks like he can’t even type in his own language properly, when I showed him the message his face went scarlet, it was " Due to a stroke I maybe have very poor French, but I sure as hell can understand it perfectly you stupid twat, now how about an apology before I ask to see your manager, bet you feel like a right ignorant idiot just now".
It was amazing the change in his attitude and how helpful he was after that 
When you first arrived in France or were moving and you were still wet behind the ears and generally clueless (!), what did you really struggle with or struggle to understand / grasp / get your head round? Healthcare? Opening hours ? Social etiquette? Socialising?
Very simple to reply, tu, vous and bisou. 
Most important difference is your tax form. n France you have to apply for it, the first year. Contact them, and they will send you one. Its an offence, not to ask and complete one. Next, you need a Carte Vitale (Green Card) ,Contact Cpam, and obtain one. They can be difficult, but not if you talk to them. Once thats done you need to register with Amelie, ( or similar) ,all on line. Easy to find. Make yourself known to local Marie, even talk to the mayor. Will simplify any probs if you are going to build/ change house size… Apply for French driving licence, if you are going to live here. A tad strenuous, but necessary. Finally, apply for Carte de Sejour, (permit to stay in france for next 10 years). For this, you need the patience of a saint, and ability to complete a lot of forms. Mine was protracted, but Macon, are notorious, for being difficult. Each dept, has different rules. My friend had to supply a translation, of his English driving licence. I did not. We have been in Burgandy for 18 years, and its great. Mayor, delivers our Masks, and we are included in OAP, Xmas lunch.
The world has changed in 18 years so some of your experience is now unrecognisable. Sometimes better, eg PUMA is now the universal healthcare and is pretty straightforward, and some much much worse - like driving licence exchange.
But just wanted to check that you have applied for the new withdrawal agreement Carte de Séjour? Just because you have a permanent EU carte de séjour which is still in date doesn’t mean that you don’t need to do this. EVERY British national has to do this. For you, as for us, it will be very easy.
No - as JJ has stated… the CdS-WA version is controlled from the centre and, apart from a few tweaks probably Covid related, is broadly the same throughout France.
If you have concerns about what to do with your UE version Cds, you can refer them to @toryroo who has her finger on the pulse with this.
Hi, yes ,thanks for checking. We applied on day one, but Macon, is a law to itself, and wheels turn exceeding slow., Regarding healthcare, know about Puma, but we don’t have private cover. 100 percent Cpam.
That’s good…, the wheels in our department have only just turned and spat out our new cards.
Yes, we struggled, because although we bought ourFrench house in 2012, I had to remain tax resident in the UK (teaching Open University EU based Arts & Humanites students online). Our main source of info was The Connexion, but later after taking early retirement ;following the Brexit vote (wasn’t allowed to work for the OU if not a UK resident), I found SF increasingly useful both for legal advice and on restoring a mediaeval French house.
I cheated, came over here at 21 in 1990 with (ex) french boyfriend, lived with his parents for a year, so I was thrown headlong into everything. As my french A-level wasn’t too far away I got on okay.
I had a good level of French but not fluent. Full immersion was exhausting.
The tu and vous thing is hard. I tended to vouvoie everyone, which probably says more about me (or what ‘you’ actually means) than anything else. I still don’t get it right every time but now I know the French don’t either, so I don’t care
Ditto for the bise, but I do miss it.
I remember opening a bank account and buying a house - I’d never initialled so many pages in my life!
Just one thing that hasn’t been mentioned: it is about when selling something secondhand, privately.
Almost invariably, without even having much detail and without seeing the item, an offer is made. Usually well below the asking price. I do get a bit jumpy when this happens and now have two replies to their offer:
(a) No, I won’t accept the offer, but to you the price is now (10% on top of the original asking price).
or
(b) If you are at the caisse in LeClerc do you make an offer for your groceries?
Either gets the same response. i.e. nowt.
The language was the big one, I had “O level” french when I arrived but it didn’t take me far. A dozen years later and I’m still struggling with my french - although it is much better, it’s far from fluent. I do try to read it as much as possible, talk to the neighbours and watch (some) french speaking TV but I think that what I really lack is good French friends to spend soirées with; I think being an old ƒǟɍŧ is a bit limiting too (s-l-o-w braincells, duff memory) . I don’t think that Covid has helped with getting to know people either: I joined a choral group but covid shot that down - and, anyway, I think it’s probably easier to make friends when you’re young.
So, I’d say to newbies - crack on with that language, every which way.