Currently on O2. Completely hate it and regret signing up for a contract. Absolutely terrible mobile internet. Can't make use of my data because there's never any internet signal. The signal is literally non-existent. Why do I have 8Gb of data if I can't even get 100Mb worth of internet usage! Insane. Forget about browsing YouTube, don't even think about that.
Used to be on 3. Absolutely terrible phone signal reception, and the mobile internet signal was completely non-existent if you're indoors (and I really mean completely non-existent) but it works reasonably well if you're outside in the open. Inside a building, you will not be able to use your mobile internet on 3 network, don't even think about it. Had to cancel my contract with them after a couple of months because I wasn't able to make calls or use the internet, so what's the point.
Next thing I'm going to try is EE. Last resort so it would better work.