Former U.K. worker and lawyer: Gardening leave for sure happens, but basic non-competes (or "restrictive covenants" as they get called over there) without any kind of remuneration are fairly common in U.K. employment contracts for skilled workers too.

Successful enforcement is almost always a question for a court to decide, and many times an employer simply won't bother trying, but provided the employer can demonstrate a business interest to protect, the restriction isn't too widely defined and that the time scale is not "unreasonable" they stand a good chance of succeeding. I'm by no means endorsing this - it simply is what it always has been in the UK for a long time now.