@kids It's a little hit-or-miss what happens when you try to activate a PAYG SIM outside the UK. The theory is that it doesn't work, but in practice sometimes it clearly does so!
I'd guess from the description that applying the top-up has probably activated the SIM, despite a lack of connection to the UK network. But PAYG accounts aren't normally enabled for roaming, which probably has to be done by Vodafone CS, although the online account may let you do it yourself! Again, setting up an online account theoretically requires connection to the UK network to receive the necessary security codes - but clearly sometimes people find it works.
I'm not sure why there seems to be this variation in behaviour - my rather non-tech picture is that sometimes the connection is much more immediate than other times, and the trick works! But, if roaming isn't enabled, then you aren't going to receive the texts with the security codes.
I'd hope that things should sort themselves out on arrival at Heahtrow, but calling 17298 might possibly be necessary. If push comes to shove, I assume that there are still payphones at Heathrow which take credit cards.
@TrudiAlexander Don't attempt a SIM swap - it doesn't work for PAYG, other than going to a Vodafone store, who will swap the old number onto a new blank SIM, always provided the old number is still active.
