Depends on your route. Brodsworth Hall is close to the A1, so is Clumber Park. Not far off the M1 there's Renishaw Hall, Felley Priory, Wentworth Castle, Hardwick Hall, Wentworth Historic Gardens.
Which route and how many stops? The fastest normal route would be the M40, M6, M74. That would usually be a one day trip but stopping for garden visits could make that at least a 3 day trip.
Alnwick Gardens, Cragside, Hardwick Hall… depending on your route of course. Assuming you’ll end up on the A1 then they are all en route just before you leave England.
As @steephill says, that's the conventional route, but it depends on how much stopping you're doing. Anything in Cumbria is worth a look. I love Levens Hall [Kendal]
It's a place where beautiful isn't enough of a word....
Tatton Park. I’ve never been before so no idea if it’s any good. It is on your way.
If you like fairies and sculptures, there’s Trentham Gardens near Stoke on Trent. I don’t really remember much about the gardens as I was too busy admiring the sculptures and catching Pokemon.
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Brodsworth Hall is close to the A1, so is Clumber Park.
Not far off the M1 there's Renishaw Hall, Felley Priory, Wentworth Castle, Hardwick Hall, Wentworth Historic Gardens.
Anything in Cumbria is worth a look. I love Levens Hall [Kendal]
https://www.lakedistrict.gov.uk/visiting/things-to-do/historical-places-to-visit/historic-houses