Hey Nau1977, to answer your first question, Sprint (and the Samsung SPH-A900 you have) operate on a digital standard called Code Division Multiple Access (CDMA). The digital standards over in Europe are Global Systems for Mobile (GSM) and Wideband CDMA (WCDMA). Your Sprint phone will not work on any of the networks over there.
Since you won't be gone long and you probably won't do anything other than make a few phone calls, I suggest you buy a Nokia 1100 or 1110 with a "pay as you go" plan. It's basically a pre-paid plan like Tracfone, but that's what they call it in the UK. Most carriers have them.
The 1100 and 1110 models are cheap enough that you can throw/give them away when you leave. Avoid buying a cheap phone Stateside and brining it over to the UK because unless they operate in the 900 and 1800 MHz band, they will not work over there. The US versions of the 1100 and 1110 are 850 and 1900 MHz.
If you Google "Nokia 1100 pay as you go" and look at websites selling them in British Pound currency, you will get a good idea of the deals available to you.
I would not buy an expensive model phone for use in the UK unless you are planning to switch your service to Cingular or T-Mobile. If you do that, your A900 becomes useless.
Good luck!
Tango Uniform.