Birmingham - Euston first class upgrade

Hi,

I've bought an advance single from Birmingham to Euston for £15 to travel on Saturday morning, May 4th. I was just wondering if anyone knew how much it would cost to upgrade it to first class onboard the train? Am I even able to do this with an Advance ticket.

All I can find on the Virgin website is that it costs a maximum of £15. Is there a set price or does it depend on how busy the train/first class is on the day?

If standard class isn't too busy then I may not bother but it would be nice to have the option.

Many thanks.