There is some wiggle room in the wording on the DBS site
"The feet of the shooter may not touch the ground mark of the firing line. The muzzle, however, must be in target direction in front of it."
Note the use of "may" and "must", one is an absolute the other is permissive.
So a clarification (even on the day of the start of the event) could be as simple as either
A. Muzzle must cross the firing line, trigger behind and the leading foot may, if needs be, cross the line.
B. Leading foot must not cross the firing line but muzzle should be across the firing line whenever practical.
What nobody ought to want is to have anyone sitting some distance back from the firing line (which might reduce the angle on steep incline/decline targets). There is no such thing as an accident. Safety is #1, etc.
Otherwise there is a possibility that in order to comply (e.g. leading foot behind) shooters would have to adopt unstable (and unfamiliar) positions which could be unsafe or cause back strains, etc or simply have to take freestyle positions kneeling/standing instead of sitting/prone.
All the best. I hope it all works out sensibly by the time the competition is underway.
BFTA Secretary 2012-2014
CSFTA Secretary 2014-2016