THY are very good, and you can always get good priced through fares from LGW or LHR to CMB via IST, on various Web booking engines in all Classes Y and J/C.
Sometimes the IST connection time is long, so you need to see what are the best days for quicker flight connections (check both ways - out and home).
Perhaps include an IST stopover?, although the new IST airport is a right old trek from the old City, the famous Pera Palace Hotel is very historic and central, and is a treat.
You may find good fares using Oman Air from London via MCT.
They are a good carrier.
Alternatively, you can get to CMB on a couple of Leisure airlines (seasonal charters) from ZRH with Edelweiss, and from MXP with Neos.
(Both have PE IIRC)
Of course there is always Emirates from LGW/LHR/STN via DXB.