Is collecting sand, for example, from a beach, legal?
Well, strictly speaking, removing sand from even a public beach, is probably illegal. Having said that, and unless you are using an excavator and taking lorry loads of the stuff, you are unlikely to have any problems, at least here in Scotland. The same applies to private land. Everything, even sand, is owned by someone, usually the landowner, although mineral rights can be a bit more complicated. The best advice I can give is, if in doubt, just ask. Permission is rarely refused. You may get some strange looks but you can handle it!