I’d like to help,but it’s not entirely clear from the details why your French lawyer needs proof of your having an NI number, or indeed where you are tax resident.
The title of the lawyer’s template - ‘Confirmation of residency..’ etc probably won’t be proved by having an NI number. Residents and non-residents can both have NI numbers.
Would a certificate of UK tax residence help - IF you’re UK resident?
Can you not show the lawyer copies of your UK tax returns filed as a UK resident?
If you’re French resident, I must admit I can’t work out why you would need to show NI numbers etc. Perhaps a little more information or background on the lawyer’s requirement for either an NI number and/or UK residence status would enable us to help?
Yes that’sto reduce the social charges to 7.5% instead of 17.2%.
I just filled form CA8421 but the first page seems to imply it’s for people working in two countries, however in the middle of it, it asks if I am selling a property abroad (so maybe that’s the right form and the first page is incorrect?)