Re: if you receive any significant benefit in exchange
"[...] perhaps they mean (only) financial or goods-based benefits [..]"
If you don't do the Gift Aid at Ely then you would have to pay for future day visits. Even two "free" visits in that time means you have received a benefit of more than 0.25% of your original donation. Using it for every weekend seems to to be questionable.
The HMRC get-out clause in that case is that anyone must be able to buy the same 12 month ticket at the same price - with or without Gift Aid.
HMRC give guidance
Sections 3.39.4 onwards lay down the rules for "free" visit Gift Aid premiums - to cover admissions to view properties and museums. These are different from the rules for tickets for performances.
That suggests that one set of rules applies to performances of a cathedral choir in the cathedral - but it is different if the choir happens to be singing while you look round the building. My head hurts. Jaffa cake/biscuit anyone?