Polo is widespread in the UK with clubs spread all over the country.
Top clubs like Cowdray Park, Guards Polo Club, Beaufort etc operate strict press accreditation policies so be sure to make sure you have the correct permission before attending. Country polo clubs tend to be more accessible but many resident professionals who attend their matches so it is also important to ask for permission to attend before turning up with all your kit.
If you haven't photographed polo before then you will need long lenses to shoot from the best areas (the ends of the pitch) but will get away with lenses under 300mm from the long sides.