Unfortunately most mobile companies have recognised that competition on other aspects of packages have are resulting in limited revenue prospects. They want to make this up with data and specifically 4G - people won't be making more phone calls or sending more messages than they already do or indeed sending photo messages or video calls (what a flop they were). But they will be using masses of more data which will vary month by month massively - even a 5Gb allowance may not be enough soon depending on how you use your phone i.e. bbc iplayer just as one example. One month you could need to pay £100s more in metered usage costs if you watch 10 or so HD movies for example.
Of all the companies three is by the far the most competitive in the 4G at the moment. Alternatively if you can manage them lycamobile have unlimited 4G for £15 - it isn't actually unlimited though as it relates back to a 10Gb allowance from their supplier after which speeds drop dramatically.
If you can settle for 3G there are other options like giffgaff, it looks like this is truly unlimited speeds don't drop but they might get a bit excited if you are using +50Gb per month consistently (check their forums).
Basically, three is the best in this regard and is a proper phone company best to use them if you can and have coverage.