Depending upon how many calls you'll be making, you might find it cheapest to buy 'bundles' which include a data allowance (even though you won't be using that allowance).
For example, GiffGaff charges 15p per minute for calls without a 'bundle' (which they call a 'goodybag'), meaning that £6 would only buy you 40 minutes of calls. However if you buy a £6 bundle then, as well as the data that you won't use, you'll get 300 minutes of call time (to be used within the month that the bundle is valid). £8 would get you 500 minutes of calls and £10 would give you unlimited calls for a month.
https://www.giffgaff.com/
Without a bundle though, Asda is cheaper at just 8p per minute for UK calls. (There are also bundles available from Asda Mobile which, again, might still work out cheaper for calls even though you're not using the data that comes with them).
https://mobile.asda.com/
GiffGaff uses the O2 network and has many fans on AB. Asda Mobile uses the EE network and I'm happy to recommend them for their excellent customer service.
I know nothing about 1pMobile though but, with an advertised price of just 1p per minute for UK calls, their service might well be worth investigating:
https://www.1pmobile.com/