Hi,
This is the OP.
Thanks to everybody for the replies and the advice. Too many to respond to individually, but here is my current situation:
I looked on the Giffgaff site but couldn't see any reference to UK (or Great Britain or England etc.) so gave up.
However, a few days after posting my enquiry, my elderly phone died so have bought a replacement.
The new phone has eSIM so Airalo seems a good alternative especially as it doesn't require a SIM to be posted.
josephbloggs, I got the idea that the AIS plan is capped to 12GB from their website. If this is not the case, then I will check with their local store as it is the easiest solution. I will use only for email checking/news/gps navigation but wasn't sure that 12GB would be enough. I will have a lot to do on this trip so don't want any unnecessary hassle.
Again, thanks to all who responded.
Jor