The website I found is
http://www.johnlewis.com/231061694/Product.aspxIt only has 2 GB of system RAM, which is the minimum amount of RAM Microsoft lists to run Windows 7 64-bit, not including running games on top of it.
What it lists as a video card is ATI Radeon 3000, which would be the absolute bottom of the ATI Radeon 3000 series. Even a 3450 would be low end for gaming. In the Radeon 3000 series, 3650 or above would be recommendable for adventure games. Don't expect much from a 3000. The computer would be adequate for email and web browsing.
Maybe someone from the UK knows of a good UK shop that sells computers. You don't like your Dell, but Dell does allow you to configure a computer from their website. Don't expect to find a good game computer in a neighborhood store unless you have a computer shop that builds computers for you.