Just idle speculation mind, but a young lad might not have the confidence to stand up to a PTI who's ragging the poor tw@t in front of his squadron/company for being on the biff. We've all seen it before.
You certainly shouldn't be presenting Gallagher Bassett with any figure, that is for your Legal Team to do. Reading their letter they will take off 15% of whatever figure is agreed away from the final settlement if you accept you contributed to your own injury. If you don't believe you contributed to your own injury then refuse their assumption but make sure the argument you put forward is a good and truthful one as if you can't agree a settlement it will go to court and you may not get what you wish for.
When settling out of court with Gallagher Bassett I instructed my Legal Team that I intended to push for the AFCS figure that I thought best met the injury I had sustained (as at the time I had been told by my Admin WO I was not covered by AFCS), and then added £10k on top. After a lot of toing & froing between offices in the same building in Edinburgh with offer and counter offer, they eventually settled for my original figure less the £10k! When I later eventually succeeded in my fight against the Veterans Agency for AFCS I received my GIP but the compensatory lump sum was withheld as legally you cannot be compensated twice for the same thing.
I'd suggest you'll get short shrift if you try and claim for a complete career as they will argue there is no evidence you would have completed a full career, that is unless you are a complete invalid and cannot work at all. Ask your legal team what a civilian would be likely to receive if they took their employer to court for a similar injury, they'll probably be able to give you an average figure and that may not be as much as you think you may be entitled to.
Oh and they haven't accepted medical negligence at all. What they have accepted is the MoD were at fault for your injury which is not the same thing.