Hi Chris, there have been many threads on this subject before (stating the bleeding obvious)
From home i use "18185" simply register with them online,www.18185.co.uk calls to Thai mobile are 1.5p a minute. There may be extra charges from your service provider though. With BT there aren't. With this service you get a monthly bill
From the car i use "local phone" http://www.localphone.com...with this service you have to top up your credit.
If you don't want to do any of that try 08448612121 then the number 0066 8 **** **** (if you are calling a mobile that is). This service is 1p a minute and shows up on your normal phone bill, again no extra connection charges with BT.
From Thailand to UK, an expat will give you the answer to that one.