If you put 'buy cheap macbook' in a search engine you will find a page of links to any special offers that are available. I don't know which version you want, and of course the Macbook Pro costs a lot more than the Macbook Air, but most offers apply to both.
For example, in the Apple store a Macbook Pro currently starts at £1099, whereas Currys are now offering one for just under £1000 - you can see more details here.So it is worth looking around, but you will usually find that the version you can buy more cheaply isn't the same one as the one sold in the Apple store - you really can't get a Mac much more cheaply.
Another option is to look for a refurbished one - these are sold in lots of places including ebay and gumtree, and of course they cost a lot less, but in these cases it's impossible to guarantee a quality product.