I'm not 100% sure, but I understood the operator also bears the operating costs for the vehicles. If this is the model Flixbus has employed in the USA and mainland Europe I don't realistically see them changing that approach for the UK.
Having to stand the costs of operating the vehicle is proving to be a problem for operators in the UK - it runs against the normal conventions of coach operation where those main costs are covered before the vehicle moves, this is a lot more speculative in that the revenues aren't predictable.
I know Linburg did work for Miki Travel prior to Covid 19 taking away all the incoming UK tourists, so wonder if that relationship has been revived. The costs of doing that would be easily understood and the revenues guaranteed as the customer hires the coach.