All lost property generally does make its way to Bristol. However, if said item remains unclaimed for a period of time (think its 6 months but not 100% sure) then said item gets distributed to charities/auctions dependant on what said item is.
Legally, ownership is never transferred, but 6 months is a passage of enough time to be relatively certain that the owner isn't coming back for it. Bristol Airport operate the same way. En route to Dublin pre-Euro days, I found a wallet with 80 Punt and no other items to identify the owner down the back of the seat, whilst fishing for my seat belt. I gave it to the stewardess, who gave it to the dispatcher. I confess that I rang the airport a month later, wondering if the dispatcher had trousered the cash. But no, they are an honest bunch. If unclaimed at 6 months, it goes to charity.
I must say, though, if I had got into the terminal at Dublin and found my wallet and cash missing, I would have at least asked. 80 Punt was then more than ^80. Without a bank card, that must have been someone's entire spending money for a break in Bristol. I'm guessing it was a youth visiting relatives, rather than someone heading to the races.