A National Art Pass. It's got me free entry to many attractions - not just art galleries but country park type places too. A prime example is the Yorkshire Sculpture Park, I was going to buy membership for them, but the Art Pass is cheaper and I can go to hundreds of other places too for free, plus half price entry to paid exhibitions at galleries etc.
National Trust membership would be another one for people who like to go to those sites, again not just 'stately homes' but places where DC can run around, bike tracks, walking trails, adventure playgrounds etc, all for about £10-15 pm so you only have to go a handful of times a year for it to be worthwhile. Plus free parking at some beaches.
I have the HRT prescription card, but for anyone who needs regular medication, the annual card is often worthwhile. People complain about the cost of prescriptions, but 80/90% are free, and for those who have to pay, no-one has to pay more than about £10 a month.
We also buy whatever is on offer for lots of things, and keep a store cupboard so we've never paid 'full price' - I'd estimate this saves tens of pounds a week, so at least £1k per annum, just by thinking ahead with grocery shopping (I know that not everyone can afford to do this, but for the majority, once you get into the swing of it, you just buy washing powder one week, coffee the next, then tinned tomatoes etc so it works out far cheaper over time than buying the same things in small quantities each week).