Specifically the term is "£150 plus any difference in fare" per flight leg.

does anyone know if this would apply to bookings made online in one class and then UUA'd over the phone?

Doing the whole holiday booking over the phone is, frankly, painful, so I'd prefer to book most of it online and then just do the upgrade part on the phone. But not if it will cost me an extra £300 for a return UUA...