I was PAYG with Virgin for years, spending £5-£10 a month until they decided they didn't want to do PAYG any more.

I went with Lebara, £5 per month, for 2Gb of data and unlimited calls and texts, and was happy, until my voice would suddenly randomly disappear for recipients during a call.

Lebara's only help was reset the phone, which didn't work.

So I went with Smarty a few months ago, £10 for 40Gb and unlimited calls and texts, but they had an offer for an extra 20Gb for a year, so 60Gb per month atm.
Never had a problem with calls, the internet speed is variable, but mostly fast enough, but can be up to 150mbs if I'm near a 5G signal. Worked perfectly in Italy, as did Lebara when I used that there, and in other European countries. I think with all these mobile virtual network operator company's, you need to see whose network infrastructure they are using. Lebara are on Vodaphone, and Smarty are on the Three network. I was worried about the 3 coverage, from their reputation, but so far so good.
With whatever network you use, if you are not getting what you are paying for, in the places you normally go (no network has total coverage), change to something else, there are many options out there. I think there are four main owners of the mobile network infrastructure in the UK, EE, O2, Three and Vodaphone, and most other company's are 'mobile virtual network operators' using one of those four company's network.