You can probably grab a decent used stereo amp for around £50 off eBay. Some brands you will be able to get cheaper than others; and if there is anyone selling as pick up only in your area, you might be able to get a good bargain. Rotel, NAD, Marantz, Denon, Cambridge Audio, Technics are some of the brands to look out for.
Same applies to speakers; you'll most like find better bargains on local pick up items because amplifiers and speakers sold by people that offer delivery, tend to sell for more, particularly if it's a bidding auction, as there is a greater number of people interested. Local pick up only auctions will get fewer interests (depending on location), so often go for less.
For speakers; Wharfedale, Mission and Tannoy are most likely the brands you're going to find for around £50.
One thing to bear in mind is the size of amplifiers, if you've not looked at any yet. Depending on where you're going to put it and how much space you have; they are not particularly small. On average 45cm wide, 30cm deep and 10cm in height. There are more compact ones available, but they are more a recent thing, so no older used bargains to be had really, and most cost a fair bit more than the average standard size budget amp.
