I've found that the difference between a bulk fare and published fare can be marginal depending when it is booked or how long you're staying.

Sometimes I've bagged a great deal in WTP with a bulk fare when I'm not staying sat night etc and I know I cannot upgrade. Other times it's been a few pounds, less than 100 to get the published fare - our corp agent knows me well enough as they screwed it up once and they had to cancel/reissue the ticket in the right bucket. They get the ticket issued immediately and I call BA to process the upgrade.