Would say its probably best to go to Istanbul/Turkey direct where possible.
Changing planes en route shouldn't cause too much bother, but it does leave you exposed to any prohibitive measures in place for country to country issues (IE if Turkey are happy to accept travellers from the UK but will quarantine arrivals from Holland for example and your flight goes via Amsterdam)
Similar scenario for something like flying to Sofia and getting the train in....under normal circumstances I'd say crack on with something like that but services could well be limited and you'd have to ensure no border issues with Bulgaria and Turkey.
In what is already likely to be a stressful trip for anyone making it due to the numerous hoops you'll have to jump through, my advice would be to keep any itinerary as simplified as possible.
On the testing front, it's quite unclear what testing will be required before leaving the UK, even to green countries.
A country might not request a negative PCR test on arrival but an airline might only agree to let you board on production of a negative test.
I'd go on the assumption that anyone travelling to Turkey will need a negative test before outbound departure, even if Turkey aren't requesting it. Coming back will definitely require further testing, both before leaving Turkey and upon arrival into the UK.
If people have the time and inclination, booking a holiday to Turkey that covers the final weekend and then jumping on an internal flight to Istanbul from your resort could be the least stressful and quite cost effective way of doing it (posted yesterday how cheaply that can be done)