+1 for GiffGaff. I’ve had one of their SIMs for years and use it for all my visits to the UK.
The big benefit for GiffGaff for visitors from overseas is they will mail a SIM to your home address and you can set it up before arrival.
I believe GiffGaff still have included roaming in the EU; however, I remember seeing something on their website about roaming included roaming becoming available only after activation and an initial period of use in the UK.
If you are visiting Ireland before the UK I’m not sure how the roaming would work.