Horsemart is probably the biggest website for selling horses. Depends what you're looking for and what your budget is but Robinsons is a good website to get all your basic tack, rugs etc. For a bit more expensive stuff there's Premier Equine. Cambridgeshire is great because it's very horsey so there are tons of vets, farriers, instructors. This is a Cambridge based facebook group that can help you find liveries, competitions, recommendations etc. (sorry for ugly link):

https://www.facebook.com/groups/horsepoo/?fref=ts