would it be cheaper to build it myself or do you know of any better deals?
As attractive as the prospect of building one yourself might sound, unless you really fancy the prospect of troubleshooting individual components, you are almost certainly likely to be better off buying an OEM machine "off the shelf" and leaving the support of the hardware as a whole to the OEM.
I've built stuff myself in the past and its generally been more hassle than its worth.... if you value your time a decent OEM machine is money well spent !
Personally, if I were you, I'd go for one of the larger OEM brands (e.g. Dell, HP, Acer etc.) rather than something random stocked by Maplin.
Finally, I'd avoid anything with an Intel i3 unless you seriously can't afford anything better .... i5 should be de-minimis, and i7 if you want a little future proofing.