Somebody on here (I forget who, sorry) posted about this last year, which we found quite useful, and I can't see a thread for this year so I'm paying it forward.
The May edition of Gardeners' World magazine (out now) contains an insert which gives you 2-4-1 i.e. two adult entry tickets for the price of one. It's a simple physical card (no apps or anything) that lasts for a year, to 02.04.25, with unlimited usage. They say that it covers 'over 400' gardens but a fair few are National Trust or English Heritage so if you are a member of those then you wouldn't be paying anyway. However, you'd only have to visit a few of the other gardens to get back your cover price of £9.50 quite quickly.