What is your destination in Istanbul - the European or the Asian side? Pegasus only flies to SAW, which is on the Asian side. TK flies both to IST & SAW (the latter from LGW) and BA flies only to IST.

I'd stay away from Pegasus as their cheapest fares might actually end up costing you more by the time you factor in seat assignments, baggage fees, etc.

TK has better IFE and food compared to BA... a thousand times better, in both Y and J.

Sounds like you don't have status with oneworld or *A, so lounge access is not a consideration. If you don't care about status, then I would pick TK even if it's 10-20% more expensive than BA. My 2 cents.

Edited to add: one last consideration is your arrival time in IST. If you're flying on a weekday, do NOT pick a flight that arrives during rush hour (3-6 pm). Istanbul has passed Jakarta as the city with the worst traffic and you may end up spending a couple of hours trying to reach your house/hotel.