They do have luxury coaches in the UK; this 2019 VDL Futura to Leger Luxuria standards is on the market for a little shy of £200k. But you would probably have to pay more than double the standard price ticket to get one of it's 31 seats, if it were to be used on National Express style service. There is also a 2016 model advertised.
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Availability is not the issue, if a customer wants a high spec 2+1 interior that is what the manufacturer will provide. Stagecoach wanted a weird convertible seat/bed arrangement for the Megabus sleeper and it was built for them.
As you say it will be the making it pay that is the issue. 31 seats in a 14m coach is basically half the standard capacity, finding enough people every day willing to pay a premium price for that on a UK scheduled service would be virtually impossible.
Even if you only provided a segment of the coach with the 2+1 layout as in the Latvian coach video at the start of the thread, unlike Rail and airlines there is no option of having sky-high expense account fares for peak business services effectively funding carting fresh air for the rest of a coach's daily diagram.
Let's not forget the snob factor, there is a significant % of the population that will not ever consider bus/coach travel and I'd bet this will have a big overlap with the segment of the population that would be willing to pay extra for a luxury seat.
Also, looking at that video, there is no way that would have worked in the UK. The standard area was full but no-one chanced their arm moving back to the premium seats once the bus was moving? In this part of the world it would be a constant battle to keep people from occupying the seats without paying. It would have to be provided at the front where the driver would be able to police it.