No, it is a fixed cost on-board for long haul flights, with the exception of the London to Australia when it is double charge for all the way through.

There is a Y to W, W to J and J to F cost. It is possible to go more than one cabin. I cannot remember the cost off the top of my head but 200, 500 and 700 GBP seems to ring a bell. Do not quote me on that as the firm amount.

It would make sense in some areas. For example, if you wanted to travel F and knew that the route was always undersold in F, then it would be reasonable to gamble on buying a Y or W cabin fare and going up from there. You can only miles for upgrade on certain fare classes. Once on-board, you can be on a real cheapie and then up from there at no additional cost.

Remember that you do not change fare basis with the onboard option, it is a duty free sale. You might get some AMEX and BAEC miles on the price of the upgrade but not the mileage, mileage bonus and tier/status points for the new cabin. Arrivals lounge rights are as per original ticket and membership status.

Slightly off topic for this thread. Payment on-board is not sensible for BAH DOH runs!