Yes,, aware that there are many testing centres in Dubai. The point I was trying (badly) to make was that it will be very clear that those PCR certificates are from a Dubai clinic. So surely they would be useless at the UK Border for proving that you have not originated from Dubai despite flying in from Frankfurt, Istanbul or Dublin.
So, once the hotel quarantine comes in for red list countries, it will not be possible to avoid it by routing via Frankfurt, Istanbul or Dublin - unless you were able to take a PCR test for the UK border without leaving those airports (which is unlikely).