Alternatively, if you are looking at standard monthly contracts, they do usually have a "soft" download limit, in that they will send you a letter or email if you go over a certain amount of data usage in a month, this is known as a FUP or AUP , which mean Fair Usage Policy or Acceptable Use Policy.
I haven't come across any UK internet providers charging for going over the usage limit.
Mobile internet will be a different story, mobile network operators love to punish you for going over monthly allowances.