Hi and welcome,
The number is a free phone number so will work from any landline or phone booth,
I also live in Manchester and a replacement card arrived yesterday and it took 6 days to arrive, so I wouldn’t worry at all yet.
If you are in some sort of shared accommodation maybe ask around about the post, if the person didn’t recognise your name maybe it wasn’t passed on to you.
Also definitely giffgaff.com accept foreign cards ,and I’m sure some other ones do too, especially if you buy a top-up voucher at a corner shop with a physical card.