Ok, more helpful than restating what you already knew (sorry!), quite a few of the larger hotel chains have helipads, might be worth picking the attraction you want to go to and then checking the accommodation lists for a decent sized hotel and giving them a call.
Also, ask the people running the attraction, they have usually been asked before.
One not to waste your time asking is Cadbury World, when I asked them I was told that they had nothing on site but there was a hospital nearby who had a pad but they weren't sure whether they'd let me use it or how long I'd be able to leave it there! No *!#t Sherlock....
