Yelp really is not relevant here. Google's own reviews are far more populated and useful, but they still require a connection even if you download Google Maps in advance.
For one day, I'd download Google Maps in advance and rely on hotel or coffee shop wifi for Google reviews and Citymapper directions (you should definitely get that app prior to arrival).
Other than that, SIM only is a much bigger thing here than in the USA so I'm sure that there are good deals around. Try Lebara or Virgin or any of the other MNVOs.