There are only 3 networks now: Vodafone/Three (recently merged), O2, EE.
Tesco Mobile and Giffgaff uses O2's network. So if you have bad signal with O2, you'll have bad signal with any other MVNO that uses O2.
You should go on the Ofcom website because they have maps of what signal is like for each of them over the UK.
I've been with Tesco, Vodafone, Giffgaff, Three, Lebara, Anywhere SIM, and Sky.
I would recommend Vodafone purely because they have the best network. I've done tests carrying several phones around with me using free SIM cards and testing the signal between Reading and Leatherhead, and Vodafone was the best. Then it's O2, and finally EE. This lines up with the Ofcom maps as well.
If you care more about customer service specifically, Tesco has the best, followed by Giffgaff. Three has the worst customer service. Vodafone customer service is not the best, however, they also have the best deals if you get a SIM via mobiles.co.uk SIM only that gives you cashback. I'm currently on £7/mo after cashback for 200GB data, unlimited texts and minutes. And I have an e-SIM installed with Sky (O2) for £1/mo for 1GB data/mo, as a backup in case of bad signal.
Also check out topcashback.co.uk as there are often good deals on there for new contracts, and you can combine it with mobiles.co.uk cashback
And finally, also check for SIM deals on HotUKDeals.com as sometimes Lebara and similar have deals for £1/mo for 100GB for 6 months.