I'd recommend a pre-paid sim (any to start) and a used phone to start off your process. You dont want to sign up for a 12-18 month contract and free phone and find a month later that you hate both.

1) Buy yourself a a prepaid SIM, probably from T-mobile.
2) Go to craigslist.com or Ebay.co.uk and get a basic quad-band Nokia phone like the Nokia E50. You should be able to get it used for about £100-£120. It is an excellent phone - I have one myself. Two actually.

Use that setup for a bit, maybe a month - and see what you think. If you like it continue. if not, you can change phone and or prepaid or contract service. You wont lose much, if anything, selling an already used phone

You will also be able to transfer your initial phone number from that pre-paid sim to a contract, if and when you decide to get one. All you have to do is request something called a PAC code from the mobile phone service company and they are legally obliged to give it to you. The PAC code enables the transfer of your mobile phone number to you at your new phone company.

All pre-paid cards on the major companies will allow you to roam in Europe if you buy a certain balance. For India and North America, your best bet is a prepaid local SIM anyway, since even with a UK contract your charges will still be quite high outside Europe.

I'd suspect that a Vodaphone or T-Mobile contract would be your best bet. Even on low minutes a £25 or so contract with most companies usually gives you a better deal than most pre-paid. And if you if you travel a lot, the roaming rates are better than pre-paid too.

But that said, I myself find that although I have a UK 02 contract, It is far cheaper for me to use a pre-paid American-based SIM when I am over there. I bought one for Cingular Mobile on Ebay.co.uk and I use that in the US.