If it’s a agri tractor (on the logbook) and because it legality exceeds 20mph (like Fastrac) it would be category “F” BUT you would have to be >17 with a category “B” pass due to the restrictions in place for with category “F” if you have NO category “B“, there is also a school of thought that says 18 or 21 because of the speed element because they are referred to as class 2 tractor (high speed).
If its down as an agricultural machine that is NOT an agricultural tractor on the logbook then if its <3500kg it is 17 with a category “B”, if its <7500kg its 18 with a category “B” and if its >7500kg it is 21 with a category “B” (also hence the school of thought above)
FYI: what it says on the tax disk “agri machine” is the main class that ALL agricultural machinery vehicles are taxed under, the logbook gives the sub-class like “agri tractor” or other which is what counts with respect to driver licensing.
Also bear in mind you also have a duty of care to employees not to let them drive things they have insufficient training or capability to operate this included knowing the laws of the road in respect to vehicles of specific weights, widths, lengths, projections etc.
Also if it ever gets stopped and VOSA view its USE as NOT being agricultural, horticultural or forestry task then the driver WILL require a category “C1” (<7500kg) or “C” (>7500kg) in order for them to avoid prosecution for not having the correct driving licence category, a category “+E” for “C1 or C” would then be required for trailers which them selves could move a vehicle out of “C1” into “C+E”, there for what the driver says when stopped can mean the difference between VOSA letting you on your way and VOSA dragging both the driver and yourself as the vehicle owner threw court.