I had to send something this week so looked into this carefully. The exclusions detail says "Musical instruments are excluded from enhanced compensation. Guitars and brass instruments are particularly prone to damage and should be packed very carefully, in a hard case" (ie regular compensation applies). https://www.parcelforce.com/help-and-advice/sending/items-we-exclude-compensation
Also (sorry I know this isn't helping at this point) "Musical instruments should be sent in a hard case. Gig bags or trapezoid cardboard guitar boxes are not suitable" (https://www.parcelforce.com/help-and-advice/sending/packaging-guidelines)
Still, a rubbish experience and a shame to break a perfectly fine bass - "It'll fix up ok" is no consolation, sorry to hear it happened at all.