Thanks for your post - here is some info that might help you.
1 - Skype or VOIP PC calling is the best way to go if you want free calls and can be near your laptop or computer. You might also want to check out oovoo.com as they offer a great video calling service too if you want to talk via video (its better than skype video calling).
2 - If you want to keep your costs down, your best bet is to get a simple prepaid sim card. They are available all over the place in the UK, just walk down the high street when you get there and you can go to the Car Phone Warehouse, Orange or O2 and they will be able to give you one. This is also assuming you have a GSM mobile phone at the moment (let me know what your phone make and model is you have and what network you use in the USA and I can tell you if you can use it in the UK)
3 - Calling home options, your best option is to use a skype service - but if you want to call from your mobile phone you can do this from services such as
http://uk.internationalstudent.com/call ... ndex.shtml4 - As for your parents calling you - if they will be calling from a landline in the USA to you in the UK they should get a calling card - try this one
http://www.icallabroad.com/usa_calling_card/index.phpHope this all helps