Just a clarification the bmi 'miles and cash' scheme...

It's not really an upgrade. You can't actually upgrade a ticket with miles at all.

However, you could claim that it is an effective upgrade.

For example:

A MAN-BGI r/t award costs 50000 miles in Y and 75000 in C.

If you want to do cash and miles, it costs 25000 + £170 in Y and 37500 + £255 in C.

What is argued, is that, effectively, the £255 'upgrades' your Y redemption ticket to C. Or, put another way, the 37500 miles upgrade your £255 Y 'fare' to a C fare.