A lot will depend upon what you're doing with your phone, Sandy. If you're using lots of data (e.g. by streaming countless YouTube videos), you'll inevitably end up paying far more than if you only use your phone for calls and texts (plus possibly checking on the weather and news headlines from time to time, as well as, say, accessing your email).
The best deals are often with pay-as-you-go 'bundles'. Asda Mobile (who I'm with), for example, offer a £5 per month bundle, providing unlimited UK calls (other than the premium rate numbers), unlimited UK texts and 3 GB of data (which is enough for checking your email and things like the weather forecast, etc). If you want more data, there are higher-priced bundles available but even a bundle giving you unlimited data (as well as unlimited calls and texts) costs just £22 per month (or £24 if you want super-fast 5G), which is far less than you're paying now.
I pay £10 per month for a bundle giving me 12 GB of data (although I see that new customers are currently being offered 24 GB). However I generally only use 2 or 3 GB through checking my emails constantly, coming to AB occasionally, checking the news and weather frequently and streaming internet radio for perhaps an hour per day. So I could probably get away with paying only £5 per month (for 3 GB of data) or £7 per month (for 5 GB of data).
https://mobile.asda.com/bundles