I’m not aware of any requirement for the tax disk to be in a specific location (only displayed on the vehicle). I remember sometime ago (probably around 20 years ago), that the police proposed a requirement that the disk be located at the passenger side of the windscreen. The motoring organizations (AA/RAC and others) acted up – pointing out that many drivers may have a valid reason for wanting to avoid a blind spot at a specific location on their windscreen. And I seem to recall the idea was dropped.
For many years, my tax disk was mounted on the windscreen in front of the rear view mirror, causing no blind spot in my screen view. I never had the old bill question its location – or anyone else point out it should not be there.