The law (VERA 1994 s.42) requires the number plate to be fixed, and not obscured (s.43) at all times.
There are no exceptions for trade plates, transporters or indeed stored off-road or in a garage.
VERA1994 is Primary Legislation. Read as normal English, it requires the number plate to be fixed and not obscured, and does not qualify that requirement.
"The law" is how legislation and common law are interpreted.
It might be the case that there is binding case law on some or all aspects of the scope of the provision - in which case that is the law.
Otherwise, the law is whatever the courts decide it is.
Road traffic legislation often specifies the scope of the provisions - some enactments apply to public roads, some to public roads and public places, and some universally.
Where no scope is specified, it is open to interpretation.
Also, when open to interpretation, it must be read in such a way as to give effect to ECHR rights. If replacing your own bumper while on your own property is a right protected by the ECHR, "the law" does not require you to display a numberplate while changing your bumper on your own property - regardless of whether or not it would have otherwise.
