I’m really sorry that I can’t help with the question you asked - hopefully someone will come along soon who can.

But I thought I’d just try to turn it round a bit - for the sake of one day, do you actually need a SIM? Rather than using Google Maps, could you not just do the old fashioned thing of taking a map with you, or printing off maps of places that you want to go to? Combined with a tube map, that really should do you for a day.

As for Yelp - maybe it’s different in London, but generally it’s not big in the UK. But again if it’s restaurant reviews you’re after, for the sake of one day, can this not be prepped in advance?

Again, sorry not to answer directly, but maybe there’s a different way of achieving what you want!