Mobile phone providers

Asda didn't offer 4G through their services, but Tesco does; whereas the parent network will.
New Asda SIM cards can use 4G.

On Giffgaff, first you will have to buy a goodybag to activate 4g on your account.
Once your goodybag expires you will be charged at 20p a day (up to 20MB).
If you go over 20MB, you'll be charged extra at 20p/MB.

Easily the best PAYG deal is 1p Mobile. The provider is EE.