You can find out from your local (UK) service provider if your phone is eligible for unlocking. Usually the first contract period (say a two-year plan) you are locked into using your phone only with that provider that in effect subsidized your smart phone purchase. If you are a long term customer, and you own the phone for more than one contract period, you only need to ask them to unlock it and they will do it -or send you to an internet link where you can do it online yourself by supplying some basic info about your phone ID (this is again an option you have in the US, but I trust UK is also efficient like that).
And yes, your phone will work once you return back to UK and reinsert your UK SIM card. As for having to buy a new SIM card 'each time' you come to Turkey.... I do not know how often you plan to come back here, if VERY often it might pay for you to register your phone -that will set you back a 100tl and some bureaucratic run-around-, but if you plan on coming say every 6-7 mos, the SIM will remain 'active' for I believe 9 months (or 6 mos at least), and even then if you ask a friend who lives here, to load a 7.00 tl basic -hold- plan, the SIM will remain active for another 9 mos (based on TurkCell provider). Alternately, to activate a new SIM card each time is about 30-40 tl. You can do the math based on your frequency of visits which option is best.
Not sure what you meant about this option to be viable only when you are away from apt. When you are at a public space, most restaurants and cafes have free WiFi for you to tap in. The problem is IN the apt where you do not have WiFi. If you are at an aparthotel, they usually have free WiFi as well. If you are in the wilderness you will need a solution -but hey, enjoy the open air when you are wandering
I agree with loz that if you want convenience and money is no object (not a big one anyway) invest on a wifi device and take it anywhere -yes they are rechargable usually so no being tethered to a wall outlet for power. As for next time you come, if your phone will not be blocked ...no first hand experience on that, sorry, but i do not think they track it consecutively. based on most of what i heard from others it is a crap shoot -some get to use it again and again, some loose it, it is probably a function of how long they hold on to information before they purge it, and how long you've been away before using the same device.
loz, i agree again that if you spent 10 bucks on a plain vanilla phone and it does the job for you, go ahead and spend 265 tl for your iPad. This was for those who are already users of an iPhone for 2 years and more.