An amp is the electronic part of the sound system--the speaker turns the electronic signal into sound--so you always have to have speakers of some kind--sometimes they are both in the same box, like a guitar amp, and sometimes the speakers and amp are seperate--

I know that this is a US company, but this will give you a fair idea of Pretty Much the full range of All-in-One PA's, in all the different price ranges.   Many of these are probably available in the UK, if not, they are pretty similar to what you'll find there.

You'll have to figure out what you need in the situations that you'll be playing in. Some units will work on battery power, some have wheels and handles and such to allow you to move them around with one hand--

The Fishman Solo, which Stringsinger mentions, sets up almost instantaneously--and has a great sound. In Santa Monica, last summer, I saw a woman get off the bus with a guitar over one shoulder, towing the Fishman behind--she staked out a little place on the mall,
and was plugged in and playing in less than 5 minutes.

In sharp contrast, there was a guy down the block with a small pa, guitar and amp that spent the better part of an hour running wires and moving boxes. She'd finished a set and was signing photos and selling CDs before he'd even played a note.

Also, since you're new to this, you should practice with your pa as if it was an instrument. That way, you should be as comfortable with it as possible before you take it out in public--and always have a backup mike.