Just in case anyone else is interested, many private gardens in London are open to the public for one weekend in June. There is an entry fee, but nothing extravagant. There is also a ballot to get a ticket to see 10 Downing Street garden.
8-9 June
Go to opensquares.org for more info.
Thanks for the heads up on this. Doesn’t match my London travel dates this spring but good to know about.