this mug from Think Geek is quite brilliant. It gets even more brilliant because they've made it to be compatible not just with Lego but also K'Nex and Mega Bloks.
The downside is that you'll have to wait for it to be shipped from the US as it's exclusive to Think Geek. The Build-On Brick Mug is $19.99 with the cheapest shipping to the UK for $9.64. In total, that makes it about £19 at the time of writing which seems eminently reasonable for endless hours of fun.