Here you go:
Cheapest: £2,163 Malaysia (so oneworld) ex-Istanbul (22/12-22/3)
Cheapest from a 'near UK' departure point: Malaysia £2,552 ex-Paris (but flying back into Istanbul) same dates. Rises to £2,825 if you fly back to CDG.
Paris flights are on an A380 with fully flat beds, IST is an A340 which may be a sloping seat.