There are maybe a dozen or so companies in UK, that I know of anyway, that convert machines for forestry. Maybe more now that I think of it.
Jas P Wilson that we use for UK made Botex cranes and trailers etc, excavator conversions, also purpose built dealership.
Caledonian Forestry that you mention, do nice tractor conversions with European cranes.
Companies like AT Osbourne and Marshalls forestry do the occasional conversion.
Forest Machine Services, A&B Services, G McArthur Forestry Services, Exc@8, RJ Fukes all do excavator conversions, A & B doing a lot of skyline winches.
Apologies to any Uk company I've forgotten here! (edit- AC Price is another, also farm machinery dealers can supply forest equipment, eg Chippenham Farm Services)
Valtra is definitely market leader for uk woods. It would seem that a lot of the farm use Valtras get a 2nd life in the woods too. You do see other brands being used in the woods, or for forwarding timber to roadside, particularly larger hp machines set up with the cabin cranes and pulling a 25 t load up a forest road for transfer to a truck. Probably the main reason is that valtra have rotating seat as standard, and option of full reverse driving controls. They are also good on clearance, they were designed with forests in mind.