There is nothing in the British Standard (BS AU 145d) or in the Road Vehicles (Display of Registration Marks) Regulations 2001 that specifically excludes raised lettering or metal plates.
The British Standard (BS) sets the standards that retro reflecting number plates must meet and states that any material which can meet all the testing requirements can be used. The number plate supplier who sells the finished product must be able to prove, if required, that the plate meets the BS requirements.
The specified Charles Wright font and that the surface of the registration plate must not have any design, pattern or texture which gives to any part of the plate the appearance of a design etc. There is no distinction made between the yellow or white reflective background or the black characters- neither should be patterned e.g. striped or chequered. The BS states that the characters must be shade of black; our advice is that this provides for 3D characters providing all other BS standards are met.
Although pressed metal plates are not excluded, the plate must not be treated in any way which impairs it’s recognition by a camera and film or any other device. The font must not be altered from the specified Charles Wright font.