I used to use about 0.3GB of data per month which was about £3 on their old 123 payg system. So a £20 top up would last me 6 months. I think that piddly amount of data is now £15 per month, which is just crazy.
The standard cheapest 1YR Three contract is £5 for 1GB, the 2GB one I am on you have to Google for. You can also get 4GB for £6, but I've done some Teams meetings on the phone now and still not gone over 1.5GB per month, so for me it is just a waste of £1 per month