National Trust costs:
£61.50 for 2 adults and any number of children
£46.50 for 1 adult and any number of children
For under 26s, membership is only £16.13, so a young family could get buy with two at £16.13 each.
www.nationaltrust.org.uk/main/w-trust/w-support/w-jointoday/w-jointoday-categories_c osts.htm
You can get 30% cashback from quidco, reducing the cost to £43.05 for a family.
Very good deal, as £43 is what you'd pay to get in one place, and there are hundreds of child-friendly NT properties (even if a grand house is not your cup of tea, they invariably have gardens, some with playgrounds, etc.)
English Heritage is another one, though not as diverse generally.
We've had Merlin Passes before (gets you into Alton Towers, Chessington, Legoland, York, Edinburgh and London Dungeons, Madame Tussauds (which is shite), London Eye, Thorpe Park, Warwick Castle, and various Sea Life Centres), got them for about £50 on ebay, Merlin charge roughly £100/head for a family (£300 for 3, £360 for 4), which is quite a lot really, yes you can go every weekend, but it does lose its apppeal after a while. And obviously it's best value if you live in Surrey/Berkshire, for easy access to the majority of the attractions (Legoland, Thorpe Park, Chessington and the London stuff). You can get the memberships via Tesco Clubcard deals too, but at the deals equivalent cost of £150, which is very steep.
And actually formal gardens, lakes and tea rooms seem somehow more appealing than standing in another long queue.