It looks like the regs only apply to new bikes at point of sale and if someone preferred their brakes the other way round there's nothing to stop them switching them.
When this was discussed at road.cc the consensus seemed to be that the bike must comply with the regs to be allowed through customs. Rose seem to have changed from jit building to stocking built bikes, which possibly makes it uneconomical for them sell British brake configs, as they would either have to stock that config, or modify each UK bound order.
By comparison, the ROI regs are advisory, so Rose can still ship continental brake config bikes there.