The AD14 from Maplins is the biggest one THEY do, you can probably get bigger ones but I don't know about in the UK as I was looking for huge breadboards too initialy. I settled with the Maplins ones though. They are big enough for most things as it happens. I have both the AD14 and AD12 along with a few smaller ones which are great for when you have small projects you don't want to dismantle yet.
Here are the dimensions..
The AD14 is... 34.5cm wide, 17cm deep by 1.8cm thick.
The AD12 is... 18.5cm wide, 17cm deep by 1.8cm thick.