Thank you very much for the reply UKtravelbear.

Looking at this further it seems that if I book an expensive economy flight it would cost me £897.37 plus 50k of my miles to upgrade that to UC (according to VA on the phone), or I can use the full 100k miles for a 'free' UC return and pay the £580 in taxes and surcharges.

This means I am paying £317 more for the Economy ticket but 'saving' 50k miles.

Not sure which is the better option.