The Rough Guide has been a stalwart of travel guides for many years and regularly produces a guide aimed towards those with physical mobility issues or parents with buggies. Today they have released their fifth edition available for free online.
The book is divides the UK into 10 geographic regions and gives both highlights and exhaustive listings within those regions. Each attraction listed gets a list of particular accessibility pros and cons such as accessible toilet or powered scooters available to hire, gives a list of information about normal and concessionary entry fees and spends a couple of paragraphs and pictures describing the attraction.
Comparing an attraction listed in the book to one I know well personally it is accurate and up to date presenting a good picture of what I would find on the ground. The PDF is available to download for free here