Hi, I did ours in May from here. I wasn't able to "download" the forms, they had to be done online. They are still in the UK passport office backlog and I'm chasing them now.
If you want to "pretend" you live in the UK then you should use the UK address, however I seem to remember them asking for a proof of address in the form of an official letter to you at the address you are using. But maybe that is only for overseas applications. If you want them sent to an alternative address you have to give a reason.
Also be aware that you will have a big problem if your passports aren't returned to your friends UK address before they are due to come over to Turkey.
Good luck whatever you decide to do.
We are having nervous breakdowns at the moment hoping ours come back before our RP's run out.