Just for basic practice what kind of PA system should I buy? I'm thinking along the lines of a shitty 15 watt practice amp for guitar.. but for a mic.

Any suggestions/links? UK stores only would be helpful too.
Almost every local musician in my area uses the Mackie 808M. It's about $700 but worth it. You pay shit and you get shit. 15watts is for children.
I was once heavily prominent on these forums from 2004-2007, let's see how long I can stay now that I'm back.
Good luck with that, I don't think you'll find what you want, you'd almost be better off with a 15 watt guitar amp at that point. It's slightly more complicated than you might think, you need to understand the way in which live sound is set up.

Also feedback will always be in issue no matter what your using, when amplifying microphones like that. Placement of monitors and mics are the most important, as well as getting levels set and having the sound "EQ's for the room".