I still have a 1 pound note, the sort that was in circulation last, the smaller one. Don't think I've got the older, slightly larger one.
The reason I've got a few halfpennies is because I made a puzzle where you showed someone the halfpenny through a hole in the bottom layer of a 3-layer wooden puzzle. You showed them how to get it out, and then asked them to do it... and obviously they couldn't. I thought about trying to market it, but once people knew how to do it it would become redundant, so I gave up on the idea and just 'entertained' others with it myself. There was never any intention to keep the coins for increased value later.
I do also have one or more old (pre-1948) 2 shilling coins with the king's head, or 10p as they later became after decimalisation. I did collect those because I was told that those with a king's head (King George VI), and particularly pre-1948, might be worth something in the future. I still have the coins, but I've never looked into their worth any further; for all I know they could be worth nothing, or they could be worth a million pounds!