Seeking advice for an international roaming SIM card selection

If you're only going to Turkey, you'd probably do better with a Turkish SIM. Here's a list of Turkish SIM providers; click on the offer links to see the rates, then check with the provider websites themselves to confirm (you may need Google Translate): http://www.prepaidgsm.net/en/turkey.html

Their list of international SIM cards is here: http://www.prepaidgsm.net/en/international.php

You can also look on their Forums for advice. Be aware that some of the responses can get VERY technical - don't be afraid to ask for clarification.

Also, check with your current carrier to see if they have deals that would reduce the price of using your UK plan in Turkey. Especially for a short visit, this is easier, and can be cheaper, then buying a new SIM. But you're right to be concerned. My friend in the US has a T-Mobile UK SIM which he uses all over Europe; as long as he stays in the EU, it's not expensive (and it has great rates for calling the US). When he went to Turkey, he found out that calls would have been £1.50 per minute - ouch!

There are many solutions for this even if you do not have any sim card on your phone, you can still make calls or receive calls from UK by getting Virtual UK phone number but to be able to use your virtual phone number, you need public wifi connection on your phone.
I do not know any provider in UK who provides virtual UK phone number but for Canada, I have two virtual mobile phone numbers from Pinger and Fongo and I have two virtual phone number from USA. How they work? You download their app on your phone, they give you a virtual mobile number free with area code you choose and when someone calls you ,they call you as if you are in UK even if you are out of UK. For example, I got a mobile virtual phone number from Spokane Washington in the USA because I have a friend there so when my friend calls me from there, he calls me as if I am in the USA but I am not even in the USA....So its the same thing for UK too. MagicJack may be offering UK phone number but not sure.

Fongo is the best for Canada and Voxox is the best for US.
Pinger:
http://www.pinger.com/tfw/

and Fongo:
http://www.fongo.com/
For USA, I got free virtual mobile number from :
magicjack and voxox.
http://www.voxox.com/
http://www.magicjack.com/magicApp.html
Again to be able use your virtual phone number, you must have internet connection and a smartphone.
But the best one is nonoh in my eyes. I have been using nonoh app for 8 years now and you can use it everywhere. You pay initial 10 euros and you call most of countries free. For example ,to call USA or Canada is free from Nonoh. Nonoh does not give you virtual phone number so if they want to call you from UK ,they can not call your nonoh but you can call them.

I hope I did not make things complicated :)
For simcard, try vodofone Turkey.
2 gb internet and 500 minutes local talking is 24 turkish liras which is about $8USD.
http://www.vodafone.com.tr/Tarifeler/esnek-2gb-tarifesi.php
Go to any Vodofone shop , and ask them 'Esnek 2GB Tarife'. but they always change their promotions so it may change....