Just measure and get the largest (capacity) battery that will fit.
I don't know if the same definitions are used in the UK compared to here, but I bought a 'Group 24' that fits in my slightly larger YM240 (YM2000).
Before that - when I bought the tractor I moved over the used battery from my wife's Subaru. It was smaller than a '24' but that worked fine to get the second four years of life out of that battery. I think the one from the Subaru is near universal across Honda, Toyota, etc.
Just don't make the mistake I made with my first replacement - the salesman told me a 'dual purpose' marine battery, intended for both starting and for running an electric trolling motor etc, was a good choice because they have less self discharge if not used for months. I discovered after a year or so that this type isn't built tough enough for the shock and vibration on a tractor, and had to replace it with an ordinary automotive battery.
