Our original route before we decide how to get to Istanbul is to get across Europe using mainly the Eurovelo 5 and 6.
We started our journey by taking the Eurostar from London St Pancreas to Lille France. You can ride to Dover and take the ferry across, but we actually found this option to be more expensive and a lot less convenient. The Eurostar has a bike carriage that you can drop your bike off at 24 hours prior to the journey with your bags and then you pick everything up at Lille station.
We generally stuck to the Eurovelo 5, excluding Brussels, and we picked up the Eurovelo 15 to ride along the Rhine for a short time out of Strasbourg. You can ride on to Basel on the Eurovelo 5 (this is by far the easier option) but we wanted to go through the Black Forest, this option does take you up a 1200 meter climb, but it is an amazingly beautiful journey.
We then picked up the Eurovelo 6 at Donaueschingen Germany and have stayed alongside through to Serbia. Excluding some times in Hungary and Serbia when the route has been uncrossable and we have had to make rash decisions to find alternative roads.
There are now several options for getting to Istanbul including the Eurovelo 13 which takes you down Serbia into Macedonia and Greece. This is probably the most appealing route but more time consuming. Based on time constraints it looks like we will travel across Bulgaria to to Erdine Turkey until we reach a ferry crossing into Istanbul. This route should be faster but does include some heavy climbs through the Bulgarian mountains.
Stay tuned for more details.