A big chunk of what you're paying will be for the phone. In a few cases, there are phone + airtime deals which are below 15 quid (but typically on Three they include 100 or 200 minutes of calls), whereas unlimited calls usually start at 10.00 as a "retention deal" if someone has been with a network for a year or more and then says they are moving to another network.
I currently have SIM only deals using
TPO (The People's Operator) - 3 GB data, unlimited calls, unlimited texts, £15 (first 3 months 3.99 and then up to 5 months free if I can encourage friends/ anyone else to join them).
Three "One Plan" which is £15 (has also been £20 and £25, but I bought it when price had been reduced) giving 2000 minutes, 5000 texts, 5000 minutes to other Three customers and unlimited internet use (with the ability to share the connection with my laptops, NowTV box, etc) I often use 30-70 GB a month.
I also have a phone + airtime contract:
Three £12 (for a Blackberry phone, 24 month contract, 100 minutes, 5000 texts a month, Blackberry services and unlimited internet traffic). Typically use 20-40 GB a month.
A lot depends on the phone you may have chosen, whether your data is 3G or 4G (some networks were charging a lot more, initially, for 4G), and whether you happened to get a good deal when you decided to sign a contract.
There are often great deals spotted on HotUKdeals.com and some networks pay out when you sign up, so check Quidco.com and TopCashBack for deals.
I had £101 cashback when I signed for a 12 month SIM only deal on a cut-price T-Mobile "Full Monty" account in 2013. It was £16/month (down from £31) and after a few weeks my £101 cam to my bank account from TopCashBack. Soon after that, T-Mobile masts were being switched to EE 4G so I could get no signal. I complained and around the 6 months time, I cancelled without penalty (as their technical changes were causing no signal). It cost me effectively nothing to use them for 6 months.
Be aware that most comparison sites will happily get £10 to £150 commission when you sign for something, but don't pass on the cash.