Certainly at LHR, the check in staff have a matrix with the cost of upgrades depending on fare class booked and (I assume) how keen they are for people to upgrade, depending on loads. I assume they have a similar thing at LGW, though last time I passed through there I saw permanent fixed price adverts for paid upgrades to MCO - don't know if the Vegas flights are the same.

I am told the prices are very much subject to change and, if I am honest, I have never been offered a price that I would even remotely consider, value for money.

The other thing to remember about the Vegas flights is that, unless an LHR plane is substituting for the day, there are only 14 UC suites in the LGW / MAN fleet, and the Vegas route is often very popular in UC.

Nick