Parking in Upper and Lower Slaughter, Gloucestershire, England
There are no car parks in either Upper or Lower Slaughter and the roads are narrow so normal roadside parking is out. There are a few small areas, in both villages, where one could park although you could easily find them occupied.
If you want to drive to the Washbourne Court Hotel for refreshments then they have their own parking for customers and I'm sure that the other two hotels would also provide parking for customers.
So what's the answer? There are no normal bus services to either village. You could park in Bourton-on-the-Water, if you are prepared to pay the exorbitant charges, and walk across the fields on public footpaths - a grand distance of about 1 mile to Lower Slaughter and a little over another half mile to Upper Slaughter. There are buses to Bourton-on-the-Water so you could go there by bus and walk. (See the parking page for Bourton on the Water)
You could do as we did and walk from Stow-on-the-Wold where there is a free car park. The distance from there to Lower Slaughter is about 3 miles. If you don't fancy walking the return journey then walk the 1 mile to Bourton-on-the-Water, it's a nice easy picturesque walk, and get the bus back to Stow as we did.