National Art Pass in UK can offer a pretty good value when you plan to visit a couple of UK cities. In addition to free admission to museums, it also covers some National Trust and English heritage sites.
Pass holder also enjoys 50% off at special exhibitions, such as Cezanne one at Tate Modern and Wildlife Photographer of the Year at Natural History Museum.
Buying a National Art Pass gives back to museums across the UK.
https://www.artfund.org/national-art-pass
Some attractions (out of 800 sites) and respective admission fees are listed here. It is not hard to hit enough spots for the investment to pay off. The pass is valid for 1 year, so you'd have plenty of time,
Btw, this is not an ad. There is no revenue generated at all. I simply think the art pass is a good option to support the museum sector while travelling.
Attraction City Admission - adult
Shakespeare Birthplace Stratford-upon-Avon £25
Courtald Gallery London £11
The Historic Dockyard Chatham £25
Leighton House Museum London £5
Horniman Museum and Gardens London £19
Apsley House London £12.8
2 Willow Road London £10
Jewish Museum London £7.5
Christ Church Picture Gallery Oxford £6
Hughenden High Wycombe £12.6
Eltham Palace and Gardens London £17.6
Lewes Castle and Barbican House Museum Lewes £5
Wordsworth Grasmere Grasmere £12
Dunham Massey Altrincham £6.6
Cardiff Castle Cardiff £14.5
The Royal Pavilion Brighton £17
Museum of East Asian Art Bath £5