The best savings are usually to be had on SIM only plans tailored to heavy users. Shop around on Black Friday weekend and it's not difficult to get a SIM deal with a monthly allowance of 20GB of more for significantly less than £15 per month.
At the other end of the pricing scale, look out for Black Friday SIMs from the likes of iD Mobile and giffgaff that get you a decent amount of data for not much more than £5 per month.