I've been learning guitar on an electric for 4 months, and am now fancying an acoustic. I don't want to spend too much, but how much difference is there between one at £100 and one at £200, similarly between one at £200 and one at £400. I'd rather spend around £100 if there isn't a significant improvement in spending more, but could go to about £300, or £400 at a push. I definitely want acoustic rather than electric-acoustic.

I know that I need to look for the word 'solid' on the top description. Do I assume that it's the same with the back and sides? ie. it's laminate unless it says 'solid'.

I was looking at the Washburn D10S which I can get for around £110. There are some versions using 'exotic' woods. eg. walnut, zebra. These are about £145. Anyone know if these are any good and worth the extra?

Any other recommendations? I'm wanting, initally at least, for rhythm, playing stuff like Oasis.

the 'exotic' woods probably have a different tone, when buying acoustics its best to go to your local guitar shop and try some £100, £200 and £300. If you can tell the difference then you will know whether it's worth the extra money.
The more you spend, obviously the better woods the guitar will have, and therefore should sound better. It should also play better and have a better construction quality.

And so you have a better idea about acoustics I will attempt to give you a basic rundown of what you need to look for.
You want to look for solid woods, which are more expensive. Cheap guitars will be made out of laminate which will not sound as good. Solid woods will sound better as they get older and have been played a bit while laminate will sound the same from when you buy it to when it breaks. The solid woods will sound better no matter what.
You should be looking for something with a Solid Spruce soundboard (the soundboard is the top of the guitar), at the price you are looking at you should be able to get yourself something with a solid top.
Having solid back and sides is not as important as having a solid top, and at the price you want you probably will not get something with solid woods in the back and sides.
I suggest you check out this thread

I really like the Yamaha FG series. And most, if not all of the guitars in that series are within your price range

Also check out some Alvarez guitars, like the AD60S. Which is a very nice guitar

Any more questions just ask
Thanks for the thread link. It's given me some ideas.

The Yamaha FGs seem really popular. I'll try and find one to check out, maybe a 730.