Yes, the Turkcell phone will work in the UK but only if you set it up for International Calls before you leave Turkey. (It can be done from the UK but I don't want to confuse you anymore than you already are.
) However, don't forget you'll be charged International rates if you make calls, or send and receive texts in the UK.
Take your old UK phone with you and buy a cheap SIM in the UK for any calls you want to make in the UK.
Also what he ^^^^ said.