If that is the case then don't use roaming at all! It is by far cheapest for you to just to buy cheap prepaids in every country you visit. It may be best to research them here on this site and order some in advance on the internet if you have adresses/contacts in the countries you are visiting. In many countries you will get an equal amount of credit to what the card costs. e.g. a €10 SIM pack may include €10 of credit, or you may even get a SIM for free!
There is so much choice here in the UK and depends on if you will make long calls or short ones and at what times. If it's a bit of everything and mainly mobiles (and not landlines) you will be calling consider
http://www.asda-mobile.com. Calls with their prepaid card costs 16p/min for the first 3 min of calls per day to any mobile and then 8p/min after that!
When you move onto the next country, come back here and check whats new. Things change all the time.