I never had a clue, the two houses, there and here are the only two I have ever sold and bought in my life. The results of being so often on the move. 
In reply to your first question - youāre responsible for calculating the gain (the UK one has a calculator on the .gov site).
As for anything else - I wish I knew.
I sold my UK house last year and did the CGT thing (none payable but I declared it) as it hadnāt been my primary residence for a little longer than is allowed (useless estate agent not putting it on the market when I asked). I duly completed the form for the French CGT (still none payable as far as I could work out) and sent it off. Last week it was returned with a note saying I didnāt need to declare in France. I posted the situation on the Strictly Fiscal France FB page and no-one there seemed any the wiser either. It may be that the property was low value and there were no gains or that it had been my primary residence before moving to France (despite the time until sale). As far as I can see, Iāve acted in good faith by completing the form and referring to it on my main tax declaration. I havenāt tried to hide or withhold anything. It may be down to tax offices not understanding the rules or I may have got it completely wrong.
Good luck with yours.
If you didnāt have to pay any CGT in UKā¦ that could well be why there was none due in Franceā¦
Whatever, you acted in good faith (as you say)ā¦ well done.