The figures are correct for your take home pay each month. SFO on french social security 6-6.5k. On UK NI, averaging 7.3-7.6k.
Unfortunately you've missed the boat now for the UK NI option. But the French figures are correct (and you get in on their excellent pension scheme). Please be advised these figures are BEFORE tax.
If you work on 6-8k per year in tax you won't go far wrong.
Any questions PM me.