I've been with o2 for several years. I have a SIM only deal that was 15gb data and unlimited calls and texts and have been paying £8.92 a month. My contract has just come to an end so I have signed up for another 12 months and now have 20gb with unlimited calls and texts for £8 a month. For anyone that's interested Google sin only deals and go to uswitch
Uswitch are good for basic type sim only plans. Remember though - to my knowledge, in most if not all these uswitch mobile deals (regardless which provider/network chosen)- it's a new connection - so new number and account. If happy to change number then it's a good way to make a saving. If wishing to retain current mobile number then speak directly to current provider re available upgrade options for your number or to request a PAC (Porting Authorisation Code- to move number to a new provider different network).
I only need 4GB data as I use internet at work and home. My O2 bill is £6 per month. SIM only. I'm happy with O2. Got free parking at the O2 arena this week and priority queue and priority lounge when we went to a concert this week.