Trimming a native country hedge out in the field is very different compared to clipping your garden hedges.

Most country hedges are trimmed by a tractor mounted flail, something like this:

So it’s no bother to trim on a two or three year cycle, which is cheaper, and much better for wildlife.
Several bird species simply won’t hang around and nest in hedges that are trimmed yearly: they need the cover of long branches that arch up and out, forming a safe “cage” of stems where raptor birds can’t snatch them.

Grazing animals are often unofficial native hedge trimmers, which may be cause for a barbed wire fence.
The cover image below shows Regenerative Farmer Ben with a section of laid hedge that has been browsed clear at the base; towards the end of the video, they show how installing a little fence allows it all to regrow.

When in the Year to Trim Hedges for Wildlife & Birds

For wildlife overall, the perfect time to cut when the branches are bare of berries, well after the leaves have fallen.
For nesting birds, it’s late February to September that matter; in most places it will only be pigeons nesting after August. Property owners can obtain a DEFRA license to control pigeons, but otherwise they still count as wild birds.
The official recommendations are not to cut hedges between March 1 and August 31, but the hedge trimming itself is never illegal, only the killing of birds or their eggs and nests:

“All wild bird species, their eggs and nests are protected by law. You must always try to avoid harming birds or to use measures which do not kill or injure them before considering taking harmful action.”

Wild birds guidance page UK Gov