I think you're wise to get a sim card in the UK. Just be sure to ask if it covers Europe. May 2022 we purchased sim cards from Tesco, I believe these were actual tesco cards, but the store sells many different cards so just be clear that you want Europe covered. Ours was covered and it worked great. We spent a week in England then 2.5 weeks in Italy (Sicily, Naples and Rome) and never had a problem, we had great service throughout. The sim cards were 5 GBP then we paid 10GBP for the data service(might have breakdown wrong but definitely total was 15gbp per phone for more data than we could ever use) . We had the person at Tesco set us up but we did the same thing in September and she showed me how to do it on one phone then I did it on the second phone, fairly straight forward. When you drop in the new sim card you then text a number provided and enter the code from the receipt for the top up service.
We had family members who got several different sim cards in Italy (vodafone, tim, another one in Sicily that I can't remember) and none of them paid as little as we did. All of the phones worked equally well. I can't remember if there was a restriction on regular phone calls in Italy but almost everyone we dealt with uses whatsapp so it shouldn't matter too much.
When we travel to Italy in March 2023, we won't be so fortunate to go through England first, so I guess we'll just pay a little more and get one at the airport or termini in Rome. It's not a huge difference in cost, I think most of our family paid between 20-35 euro for their sim cards. One note of warning though, I found that when asking at each place for the cheapest price, if the price sounded higher than I wanted, I kept asking isn't there something less, and usually there was. It's not that they're trying to rip you off, they just think you want a higher plan than you need.
Good luck.