A good variety of networks have deals on data-only SIMs - use our comparison tool to see which ones offer plans that fit what you're looking for.
Look out for networks with additional benefits that would suit you, too. Giffgaff is nice and cheap, for instance; EE has the UK's widest 4G coverage; and O2 gives you free access to its Wi-Fi hotspots, a very useful extra for using your tablet out and about.
You can get data SIM deals from networks such as:
EE
iD Mobile
O2
Three
Vodafone