It's worth remembering that there are only four 'real' mobile phone networks in the UK. (i.e. different sets of transmitting masts). They are Vodafone, EE, O2 and Three. 'Mobile virtual network operators', such as GiffGaff, Tesco Mobile, Asda Mobile, etc simply 'piggyback' onto those 'real' services.
For example, there would be no point in someone who was getting a lousy signal with O2 moving to GiffGaff (or vice versa) because GiffGaff uses O2's transmitters. Similarly, customers of Vodafone who move to Asda Mobile won't get a better signal because Asda Mobile uses Vodafone's transmitters.
The situation with Virgin Mobile is a bit more complex though. If you've got a 5G phone with Virgin Mobile, it's currently connecting to the Vodafone network when you make or receive calls (or use the internet). However if you've got a 4G phone with Virgin Mobile, it's currently using the EE network.
You'll, no doubt, have noticed my use of 'currently' in my previous two sentences. That's because things are due to change fairly soon. Virgin Media has merged with O2 and will therefore be transferring both of Virgin Mobile's 5G and 4G services across to O2's own network.
So simply sitting tight with Virgin Mobile might be all that you need to do, as your phone will soon be getting signals from a different set of transmitters.
If you can't wait that long though, you'll need to ensure that you avoid switching to any network that uses Vodafone's transmitters if you've got a 5G phone or to any network that uses EE's transmitters if you've got a 4G phone.