Going from the VS website You can’t use miles to upgrade from PE to UC if you have a K fare bucket (see text just below ‘our seaaons’)

https://www.virginatlantic.com/gb/en...d-flights.html


The codes used in the fare rules also don’t cross over to the codes of fare buckets. I’ve just done a dummy PE booking and it shows the fare bucket as K but the fare conditions code starts with U. As far as I’m aware VS does not do instant upgrades though many US airlines do so the use of UG in yiur fare rules is likely to be a coincidence.