You could save up and upgrade to the 360S if you wanted to stay with a 360? Otherwise, find a bigger HDD (which may be more difficult, as you likely have the older one)...
250GB Slim console is about £170 from Amazon(I'd recommend going for a new console, over preowned), for example... You can easily get a Data Transfer Cable for about £12 from Amazon, to transfer all data over, so don't need to worry about losing anything... So yes, this option is about £180 altogether from Amazon... Or Game have some bundles starting at £200 for the same console, plus a couple of extra games too... ShopTo.net also have a few bundles as well...
You can also look for 120/250/320GB hard drives, but I don't think there are any sold by places I'd recommend (external sellers on Amazon, mostly, or eBay), so you could try this route, but it's kinda taking a risk with "will it, won't it work?".
Does this help at all?