Slide scanning services in the UK seem to be between 30p and about 75p from what I've seen after a brief search (prices go down for large quantities)
You could buy a slide scanner, which cost about £50 but you would be limited to only ever being able to use it for scanning slides.
http://www.amazon.co....qid=1273579142&sr=8-5
Or you could buy a flat bed scanner with slide capabilities, which although a little more to start with can be used for scanning normal paper which widens it's usage potential.
http://www.amazon.co....id=1273579142&sr=8-11
Keep in mind that scanning a large amount of slides yourself is going to be time consuming, so now you have to weigh up how many slides you have against cost and your time to figure out if it's best to get them done for you or buy the equipment yourself.