I was taken to many National Trust houses and gardens during my childhood, and have had my own National Trust membership for most years of my adult life. Looking through my list of direct debits recently, I briefly considered the value of my National Trust membership, and concluded that this is thoroughly worthwhile. First, the National Trust is an organisation which does a great deal to preserve the English countryside, including many historic houses and much beautiful scenery. Therefore, even if I didn't not use my membership fully, this would feel like a useful charitable donation. Secondly, if I make even a few visits to National Trust houses or gardens each year I will soon save myself more than the cost of membership. Thirdly, because I can visit houses for nothing, if I want to make a flying visit somewhere, or go repeatedly to the same house or garden it is possible to do so without feeling that I am wasting money on entrance fees. (I was taken to a National Trust place near my grandparents dozens of times during my childhood, and never tired of it, and I quite often visit my nearest National Trust garden for a walk.) Fourthly, I find it educational to visit National Trust houses and gardens, always learning something in the process, whether about the history of the family involved, or the plants in the garden or the china or furniture or paintings.
National Trust membership also makes a sense for holidaying in the UK - English membership was, last I heard, recognised by the National Trust of Scotland - as wherever you are staying, you are likely to have some places to visit nearby. In the Lake District recently we found that some car parks were National Trust ones, helpfully saving members a few pounds here and there on parking fees. Alternatively, if you are staying at home for your holidays, you can use your membership for some inexpensive day trips.
Finally, many of the National Trust properties have cafes and restaurants, with good and reasonably-priced lunches and very nice cakes and ice-creams, with an apparent recent emphasis on local produce.