They are actually legal if done correctly in theory.
They actually changed the wording of the law last year due to the ambiguities - so basically any stick on plate is illegal.
But, using the DVLA guidance of conformity there was a grey area.
However, places that are licensed to sell plates don't offer the service, and therefore by proxy are illegal.
"non-compliant with BSAU145d, in accordance with DVLA regulations. Our plates are reflective and we use the legal font (unless a different font is chosen). Bendy, flexible, stick-on (self-adhesive) plates are not made of solid acrylic and are more like a reflective sticker that can be placed directly onto the bodywork"