Giffgaff's website is a bit inconsistent. I think we'll probably try to get a GiffGaff esim first as having a local number would be handy at times, but if we have problems we'll just use Nomad. If GiffGaff doesn't roam to France, we'll get Nomad when we get there. Nomad has good data only UK and France packages, might cost a a little bit more than GiffGaff but not enough to make a difference. The prices are pretty good these days, and esim should make things a lot simpler. I'll purchase the GiffGaff or Nomad UK package a few days before we go with my Wise card, and activate on arrival at Heathrow.
Thanks all for the advice and help 🙂