They already do - they pay for it now!
All of it? Why? I assumed it was partly a Westminster political bung for Scotland. In the event of independence England/Wales/Northern Ireland government would presumably be happy with the Caledonian sleeper services staying if they ran as an open access service within England.
The best option when HS2 phase 1 fully opens (2031 to Euston?), would be to configure the Caledonian Sleepers with 18 of 75 coaches transfering to run the Night Riviera. That would reduce subsidy for Scottish service and keep the Cornish service going longer. Hopefully by 2060 London - Penzance will be fully electrified with a day journey time under 5 hours making the sleeper redundant.