A real grey area, as mentioned above. There seems to be little to endorse the legality of pressed plates, but equally, little to confirm illegality.
As motorists, we have a number of disparate bodies concerned with the conformance of number plates; the DVSA ( nee VOSA ) , MOT stations (who may choose to interpret the 'plate specifications differently) plus the ones we are most likely to come into contact with, where there is a 'dispute', as to the plates legality; the Police.
imho and with the balance of probabilities, it comes down to this; as long as your number plates comply with regard to size, spacing, font, background, appearance of BS mark (though if the document mention below is referenced, equivilant EEA standards are acceptable ?!? ) etc etc, then I would doubt there would ever be any issue.
The issue may arrise however, when an 'officer of the law' believes the pressed metal plates per se are illegal and slaps you with a fixed penalty notice and a £60 fine. It then fall down to you to reject the FPN and have your day in court to challenge otherwise ! .......... What ever happened to the premise of ' innocence until guilt proven ', or does that not apply to ones wallet !?!?
On hunting around, the only definitive confirmation of acceptability of pressed plates is for pre 1973 vehicle as described here:
http://www.legislation.gov.uk/uksi/2001/561/schedule/2/made
which, for later registered vehicles also references reflectivity both in 'retro' and 'reflex' forms
Good luck Steve which ever way you decide to go !!