According to the National Trust's Wicken Fen website:
Grazing animals are essential in developing vegetation in new areas of the nature reserve. The animals help to keep the landscape open and help wetland and grassland plants to become established.
We chose these breeds to graze the land at Wicken Fen because they are more than capable of withstanding the rigours of life on the fen throughout the year and thrive on the available vegetation.
Unfortunately, I have not seen both types of grazing animals when I was at Wicken Fen NNR in 2022. I did not know about the Highland cattle but our guide mentioned the Konik pony that was why we tried looking for them during our tour but to no avail.