Thank you gentlemand, if I exchange on payeer first, could I sell the bitcoin for more GBP than the debit card website would exchange for?
Hmm. I'm not familiar with Payeer but I've just had a quick look at their website and I can't find any GBP option on there. GBP in general is a nightmare compared to other currencies because of the lack of banking. Most payment processors won't touch it for fear of losing their banking. Maybe I'm not looking in the right place.
In general the debit cards will be market rate. I've just got a Bitpay debit card and their current exchange rate is £2133.
Preev.com's exchange rate, which is balanced from several sources is £2122.
The next easiest option for a Brit to turn BTC into GBP would be to sell to a Localbitcoins trader by bank transfer. You'd get more selling to the public on there but I wouldn't bother myself.
The current highest price paid by a trader buying off you is £2140. Personally I'd stick with the card for the convenience factor myself.