This is a really fun event and the only one of its size within London. Hosted at the historic Hurlingham Club, it is the base of historic polo in England. But the organizers have taken the traditional game and given it a 21st Century "Extreme Sport" edge. It is played on a smaller field and is faster flowing than most set ups. Moreover, you can get to it by London transport and there are plenty of bars on the grounds to make you glad you don't have to drive home.
All three days are fun but different. The Friday is geared to corporate entertaining and there are lots of suits about. Men would be advised to don a blazer and possibly a suit and tie if you'll be meeting clients. Women don't need to dress for work but a nice spring dress with flat shoes or wedges will look the part between formal and fun.
Saturday is more relaxed party atmosphere. Think "picnic in the park". If the weather is nice, T shirts or polo shirts and white jeans would be a comfortable option. Leave the ties at home on this day.
Sunday is more of a family day but similar to Saturday for dress code and a lot of fun if you don't mind a few ankle biters.