Yes, fencing is very costly especially because cows are strong. I grew up on the countryside and learned a couple of things. Fencing not only needs to be built but also maintained. You can also use electric fences but that's basically the same, and you need to run power.

Cows are stubborn and especially in summer, when they found out that the water cools them down, they get into the water. But often they can't get out, at least how N/W-European pastures are. So you get a tractor and pull them out, but then they learn that :-)

What actually is necessary, is that there's enough shade. But trees need to maintained so a lot of farmers cut them down.

Note that my knowledge is limited and perhaps already out of date.

The cows graze in parks shared with people. You don't want to fence around the people, just the cows. This is an example of a field that the cows visit regularly:

https://maps.app.goo.gl/iYbCeJu8fvrmuAXz9

Would cheese off a few people in the area, since along that stretch of river there's a popular riverside picnic spot and a mooring area for houseboats.

(Also, it's a common, which in England means massive piles of red tape to build anything on it)

Pretty sure they might be cheaper that £10k a year.

Fences are useful, but if a cow truly wants through a fence, they can and will just push through it and snap it. Electric fences and barbed wire can make them a better deterrent, but even then if the grass on the other side looks tasty enough they will push all of their potentially 2,000+ pounds against that fence until it tips over or snaps.

The cows are on public, multi-use land.

https://en.wikipedia.org/wiki/Common_land