Parcel Force do carry section 1&2 firearms a few of the gun trade uses them, need a business account and RFD to RFD only. Prices dearer than TNT.
Unless some things changed recently, TNT have not took on new accounts for years but do carry for existing customers business to business only. A good 95% of gun trade uses them nationally.
Ring up national hub last 10 years to arrange collection "we don't carry firearms" Ring local hub who know you, hunky dory taking details of delivery address, size, weight, goods S1 or S2 and value.
Then theres the problem of insurance payouts, lucky if get 1/3 of value after years of court battles trying every trick in the book as a get out clause.
There are 100's of couriers that beg for your business, will collect and deliver even same day. Soon as insurance mentioned they all decline.
For smaller items (parts) of higher value very few in the high class gun trade don't use Royal Mail Special Delivery. Reason at least they will eventually get £2500 back rather than nothing.