Not sure about XC but the TPE and Virgin contracts are with Carlisle Cleaning Services, I believe that the current rate of pay is about £6.50 p/h. The pay is pretty much the same with all TOCs as the cleaning services are contracted out and there are only really 4 or 5 cleaning contractors big enough and experienced enough to tender for the contracts and the TOCs tend to try and get the work contracted as cheaply as possible.
LU pay £8.55 but they are governed by TfL and therefore pay the London Living Wage, although they don't really have in-service cleaning other than litter picking on the Circle Line.
Not sure where they are based but it is generally at a terminus as they need to dispose of litter/replenish cleaning supplies.