Two points here:
1. Technically, it's illegal to listen to aircraft radio without a licence. Not that it's very likely you'd be "done " for it, but be aware.
2. For £100, you won't get anything new that's useable for listening to airband. It pretty well has to have digital tuning to be any good.
Look out for a second hand one - any of the well-known brands of digital/synthesised receivers (or scanners, but they tend to be dearer and to cover a vast range of frequencies where you won't hear anything interesting).