"UK taxpayers who fill in a Gift Aid form can visit again for free the following year."
I am always puzzled by this. Ely Cathedral has a similar scheme where you can pay a small premium on the day ticket price and elect for Gift Aid. That then gives you free entry throughout all the following 12 months.
Yet HMRC are reputed to invalidate tax claims for donations to registered charities if you receive any significant benefit in exchange for the donation. IIRC a businessman gave a charity a large donation - and HMRC ruled that a free ticket to an event for him was OK - but two tickets was too much.