Perhaps just trying to avoid a breach of Rule 160 of the Highway Code
Waving to other drivers in the UK could be illegal and result in a fine of up to £1,000 if witnessed by a law enforcement officer. Waving could be considered a breach of Rule 160 of the Highway Code, which states that drivers should keep both hands on the wheel or handlebars as much as possible. Other actions that could result in a fine for breaking this rule include flashing your headlights or giving a thumbs up.
Yeah the Highway Code says a lot of things. I think it's about 4 times bigger than when I passed my test. Makes me wonder how much of it is strictly necessary.