Pension advice

I worked in the uk for 10 years in the 80s/90s before moving to France and during that time l opted out of SERPS. I therefore have a private pension still in the uk and am looking for advice from anybody in this situation. For example,if l wanted to transfer the whole sum at some point, what percentage would l lose in taxes in France? Has Brexit changed things? etc… Thanks for any help/advice. :blush: