We had a similar situation in France 5 years ago - I suspect the rear plate, which was stuck on with tape, had snagged on bushes on an aire, but we didn’t realise until a French motorhomer pointed it out at lunchtime, a couple of hundred kilometres south. We were lucky no gendarmes had noticed. The French car accessory shops couldn’t replicate the British format on their computerised systems, and although the old style pressed metal plates, using a die, seemed to be a temporary solution initially, unfortunately at least one of the letters in our registration number is not used in French plates.
We eventually improvised a replacement with sticky plastic sheet from a diy store, tracing the form of the letters from the front plate. It won’t have been legally compliant, but didn’t attract police attention over several weeks in France, Spain or Portugal, and more impressively was accepted by ANPR cameras at campsites.
Fortunately, our son was flying out to join us in Portugal, and we got our dealer to send him a duplicate plate.