Thanks. Is their European roaming charged the same as for o2? i.e. £3/MB? I cannot see their data roaming rates on giffgaff - pricing

That tariff hasn't been announced yet.

So far, roaming data use has been barred, until the free data ends

The new domestic data rate for after the 25th of May has finally been announced:

It will be charged at 20p/MB. A daily cap will be applied after 50p have been spent (i.e. 2.5MB) which is valid for up to 30MB. After that you'll be charged again at the same 20p/MB rate).

Any idea whether it is possible to activate a Giffgaff SIM outside the UK?

No, or probably not.

They don't accept foreign debit or credit cards, or post to foreign addresses, and there is some ambiguity whether they also have IP address measures.

I was told the IP part a few months ago by a support agent in respect of topping up myself, and that they might be reviewing this later. So at that point I might have to either set up automatic top-up in advance or call in with a voucher number, options which wouldn't apply to activation. However, on the forum, some people have seemed to managed from some countries more recently,though others still could not.

However, I could arrange something (including bonus credit for the SIM), and maybe post to you, either active or to be done on arrival here (giffgaff or O2 vouchers can be used). I'll also contact support to see if there is any update on the position.

Giffgaff is raising its call prices to 10p/min and texts to 6p on 6 October:

Changes to call and text prices - The giffgaff community

It's still a good deal, though.