I've seen this brown beetle, which was about 2cm-3cm long (I guess) near Loch Lomond (Scottland) a few days ago.
What kind of beetle is it?
It's not a beetle but a shield bug, probably a forest bug (Pentatoma rufipes): Wikipedia. The Collins Complete Guide to British Wildlife describes these as widespread in the UK, with adults seen from August to October.
Beetles are from the order Coleoptera, bugs ("true bugs", rather than the informal use of the term to mean, basically, any creepy-crawly) are Hemiptera. Beetles have two pairs of wings, but the front pair is hardened and acts like a shell and only the rear pair is used for flying. Bugs have either one or two pairs of wings. The front half of the forewings is usually hardened for protection, but the rear half is unhardened; if there's a second pair of wings, they're usually unhardened and all wings are used for flying.