Friesians are not common in the UK, though in the 19th century they were popular in the funeral trade. There was probably a two way business: horses into Britain and horses going to Europe for slaughter e.g. on the “Antwerp Boat” in Tilbury Docks that used to take unsound working horses from London each week.
Do have a look at “Dales Ponies”. They are a Yorkshire breed, sadly rare today but once the all rounder used by farmers and traders for work and leisure. They can look absolutely splendid with plenty of hair and feather. Dark colours are more common so they can do a mini-Friesian rather well.
But I suspect it is the usual thing of having whatever horses are available to hire from the specialist trainers and assuming all horses are just horses.
I once found a very small yard on the edge of some public open space in London so, being nosey, when I saw a human there I went in to talk to him. “Goodness” I said “I would never have expected to see two Friesians here”. He responded, “And I would never have expected to meet someone here who knew they are Friesians!”