It is covered under the construction and use regulations…
These Regulations revoke and replace regulations 17 to 22 of the Road Vehicles (Registration and Licensing) Regulations 1971 and regulations 18 to 23 of the Road Vehicles (Registration and Licensing) Regulations (Northern Ireland) 1973 and instruments amending those regulations by making new...
www.legislation.gov.uk
Basically you should be OK anywhere as long as it is clearly visible, easily readable, vertical and lit at night.
There is a bit of a gotcha here in that the legislation requires that:
(5) This paragraph requires the plate to be fixed—
(a)vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practicable, and
(b)in such a position that in normal daylight the characters of the registration mark are easily distinguishable from every part of a relevant area having the diagonal length specified in paragraph (6).
(6) The diagonal length of the relevant area is—
(a)in the case of a mark having characters the width of which is at least 57 millimetres, 22 metres,
(b)in the case of a mark having characters the width of which is 50 millimetres, 21.5 metres,
(c)in the case of a mark having characters the width of which is 44 millimetres, 18 metres.
The 'relevant area' is a diamond shape at ground level with equal axes and with the front corner positioned directly under the numberplate. Given that it's impossible for the characters to be easily distinguishable from directly below the numberplate (i.e. reading edge-on), strictly, this requirement is impossible to meet and hence the only (strictly) legal numberplate position is mounted in the space specified by type approval legislation. AFAICT, this is Regulation (EU) 2015/166, which provides different criteria depending on whether the upper edge of the plate is above or below 1.2 metres from the ground. However, that regulation cites others -- and I didn't have time to go down that particular rabbit hole!
Basically, I suspect that you'll be OK provided that the numberplate is readable from any reasonable standing position in the 'relevant area' and lit with appropriately marked fittings. HTH