FI is indeed a normalised measured of aircraft life usually scaled to 100 FI units. Consumption depends on a number of factors include role fit (ie tanks, weapons, pods etc), fuel and sortie profile codes (SPCs). Normally, the FI per sortie is calculated using the Mid-Sortie Mass hence the rather tedious RAF F700 logbooks which require all sorts of figures and codes. I'm not familiar with Bulldog fatigue but some aircraft will have different fatigue lives for different components ie wing, fuselage and tailplane all have their own fatigue life which needs to be managed. I suspect, without any real knowledge to back this up, that even if a Bulldog was relatively high on FI, ie close to 100 FI units, that civil use is much less demanding than that dished out by the UAS' so you could probably get decent number of flying hours from an airframe even if short on FI. High FI airframes need to be carefully managed but as long as fatigue consumption is low they can soldier on.