The relevant part of the applicable statute is reproduced below:
Statutory Instrument 2001 No. 561
The Road Vehicles (Display of Registration Marks) Regulations 2001
PART 1
VEHICLES REGISTERED AND NEW REGISTRATION PLATES FITTED ON OR AFTER 1ST SEPTEMBER 2001 (MANDATORY SPECIFICATION)
1. The plate must be made of retroreflecting material which, as regards its construction, colour and other qualities, complies with the requirements of -
(a) the British Standard specification for retroreflecting number plates published on 15 January 1998 under number BS AU 145d[13], or
(b) any other relevant standard or specification recognised for use in an EEA State and which, when in use, offers a performance equivalent to that offered by a plate complying with the British Standard specification,
and which, in either case, is marked with the number (or such other information as is necessary to permit identification) of that standard or specification.
2. Where the registration mark is displayed on the front of the vehicle, it must have black characters on a white background.
3. Where the registration mark is displayed on the back of the vehicle, it must have black characters on a yellow background.
So, it has to be retroreflective, and it has to either (a) comply with BS AU 145d or (b) comply with an equivalent EEA standard (TuV would perhaps be considered equivalent in a court of law).
Nothing in either the statute or the standard precludes the use of pressed plates. It does not necessarily follow that any particular pressed plate is legal (e.g. BS AU 145d approved) as a result of this, of course. All herrings are fish, not all fish are herrings. It should be perfectly possible to manufacture a pressed plate that complies with 145d; whether or not anyone has actually done this is a separate matter.
Chris