My understanding is that the visa resets when you leave the kingdom. If you ever wanted to extend your stay it’s only a visit to an immigration office and your all set for a further 30 days on top if your initial award of 60 days
I always take cash. The higher the note value the better exchange rate. Try to get £50 notes but they must be clean no ink scribble or creases. If your changing a considerable sum it’s possible to negotiate a better exch rate than offered