I'm planning a first trip to England this coming month of October and was hoping to visit a couple of gardens in the Cotswolds region. Interests are in conifers, japanese maples, and woodland type gardens.
If you are interested especially in trees try to get to Westonbirt Arboretum. Bath itself has a small Botanical garden in Victoria Park and many superb specimens of old trees in other parks in the City, though not especially conifers or maples. Also Prior Park landscape garden for a good example of the English landscape movement. There are numerous fantastic gardens in the Cotswolds, eg Kiftsgate and Hidcote, but they are not famed for woodland; that's not really the style of gardening which is predominant round this area. Look up also the National Trust and the Yellow Book for gardens open. You could visit a garden evry day of your visit and still hardly scratch the surface of what's here. I hope you have a great trip - I live in Bath and am grateful for it every day.