Most of the coins were 2p pieces. I noticed a few from the 1970s, but nothing really old – nothing pre-decimal, from before 1971, never mind "dating back centuries"!
Just for the record: I've never heard of these things being called 'wishing trees'.
It's entirely possible that people make a wish whilst hammering-in a coin – it makes a nice story, anyway – but I'm slightly alarmed to see people linking to this page as evidence of the practice. The photo proves that people have inserted coins into a dead tree, but nothing more – claims about their motivation are mere speculation!
I have heard that this tree was populated by geology students on field trips to the spectacular limestone landscape; I've no proof of that idea either, but it seems as credible as the 'wishing' suggestion.
[Further detail for those visiting from StumbleUpon, Pinterest, Reddit, et al.: this dead tree has not been here "since the 1700s" : that's ludicrous.]
Better On Black?
[Image reached no.180 in Flickr Explore on 05/11/08! Thanks!]