Booking has now opened for the Open Garden Squares Weekend on the 11th & 12th of June, when a number of gardens not normally accessible by the public will open their gates, and others will be holding special events to encourage visitors. Three cemeteries - West Norwood, Tower Hamlets and Bunhill Fields - are taking part, as well as several former burial sites which have been converted into public garden spaces. I'd recommend St George's Gardens and St Dunstan's in the East if you haven't visited.
Tickets are £7.50 for the weekend if bought in advance (£10 if you buy them on the day), and can be bought in a number of ways. (The Capital Gardens website is not especially user-friendly, so I'd use one of the other ways if you can.)