Some information about our club before answering your questions.
Our club, active since 1968 does all the formalities for customs purposes for bikes including parking when you are abroad, giving technical reports for air freight forwarding,etc. and has done so many operations in customs inside and outside Turkey all legal on paperworks and bureacracy.
Rules has been changed since 22.april.2011 in Turkey according to the EU. meetings and discussions.
UK registered bikes can enter Turkey, for 6 months, and this period is 1 year if the owner is retired and has a letter mentioning this point and given by Turkish Consulate in the home country.
To depart Turkey and leave a vehicle behind is not possible legally.You can leave it in customs parking area, or a confirmed company,association,etc. for 6 months with a letter saying that you will get it back before the time expires.During this period noone can ride it.
You can leave Turkey with your bike behind without any formalities and noone can stop you in the bordergates.But you will be faced with some fines at your revisit which will be more than the value of the bike.
Customs authorities will block and sell your bike wherever its found, and you will have no right on it.You will not be able to enter Turkey again until all fines are paid.
With the letter of authority given by the owner, anyone can get the bike inside and outside Turkey.Enter and exit formalities are also done on the computer so its not a must to have the exit stamp on your passport.Enter and exit stamps will be done to your number plate on the computer.
Attention, you are the only one responsible to all accidents,punishments and losses to third persons even your bike is not exited by the owner.
If you can find a non Turkish citizen customer in Turkey to buy your bike, you can sell it without any taxes.
No need to fill a form if your number plate doesnt start with x,q,w letters which are not in Turkish alphabet and the system accepts your number plate.Very soon this problem will be over too.
Website for up to date information.
Wish you all the best.