I'm not that familiar with Lancashire, but if you check out the National Gardens Scheme site this lists both private and public sites which are open to the public.

Can I persuade you also to come down to Kent/Sussex (South East England - about an hour from London, so maybe you could combine with Chelsea ? )which I am visiting this summer There is a huge number of gardens, notably Sissinghurst and Great Dixter which are internationlly reknowned and listed as in the top 10 influential gardens of the 20th Century - I am really excited about this !!!

Enjoy planning your holiday !
PS the web site also lists accommodations (under links - which may be helpful)

Here is a link that might be useful: England: National gardens scheme

Hi! I just went to England last week...I bought the most amazing book at Waterstone's: 'The Gardener Lovers guide to Britain' by Kathryn Bradley Hole. You can probably order it off of Amazon.comUK. Awesome book. I would recommend it! David Austin Roses is in Wolverhampton...sort of central if you like english roses. Anthing in the Cotswolds...more south though. Good luck! Susan