IO540: Not much avgas in Greece. Corfu LGKR has avgas, then Mykonos LGMK. Corfu would thus make a good final stop on the way to Turkey.
Alternatively Sitia LGST has avgas and is also a port of entry, at times of the year anyway.
The route to avoid the Alps and eastern block countries for a UK-Turkey trip I recommend is to fly via Southern France (i.e. Cannes LFMD) then via West and Southeast Italy to Kerkira LGKR in Greece. Then from Kekrira eastwards to Istanbul the route takes you via Thessaloniki LGTS which is also an international airport with AVGAS, so you have two options for stopover in Greece.
So you could easily do S. Italy - Northern Greece (Kerkira or Thesssaloniki) and then enter Turkey via GOLDO intersection.
If you go to Istanbul plan to land at Sabiha Gokcen LTFJ but most likely this is where your friends will ask you to come.
Yes, for N'reg and/or VFR, Turkey need 24hrs prior notification for entry clearance.