Hello all, hopefully you can help me!

I have an unlocked phone (recent model from vodafone) that was used in Australia. I am told this will work in Europe. I plan to take it with me to Europe for my 8 week trip.

I will be in Greece for 3 weeks, Italy for 2 weeks, and London for 4 weeks (in that order) with short trips (3 days) to France and Spain. Does anyone have a similar experience, and what did you do?

What is the cheapest option for cell phones. I was planning to buy a sim card with the pay as you go option when I land in London on my way to Greece in the begining and keep the same one throughout the trip in all countries. However, from recent posts I have started to understand that some people buy sim cards in each country they visit - is that a better option?

Things I would like to use the phone for: safety, keeping in touch with travel companions (call to meet up, etc), calling ahead to verify reservations, and sometimes recieving calls from home (u.s.). I would use a calling card to do the majority of my calls home because I understand this is the cheapest.

So my question is should, I try to get a SIM card for Greece, Italy, and UK, or just UK? If I just get one from UK what is the best SIM card from UK (or Greece, b/c that is my first stop) that would work for the entire trip.

Can anyone help? Please?