65 k€ = 3358 € per months (on "12 mois") after all taxes, if single living alone.
85 k£ = 4712 £ per month, after all taxes.
Also, 65 k€ is a fairly good end of career in France whereas 85 k£ is a good middle experience in London (and you can get bonus, pension and perks on top).
That said, if you're thinking of dropping £1500 on a 1 bed you'll have no problem IMO. I was paying £1400 for a 2 bedroom with garden and huge living room in Leytonstone only a few months back. As long as you're not planning on living in Mayfair or Kensington that's a nice budget to work with.