Non deciduous trees turn brown then back to green again.

[Peasgood]

At that time of year I would be suspecting frost rather than herbicide and I can't think of any herbicide likely to be applied at that time of year or one that would do as claimed.

[Vedhoggar]

There was subzero temperatures in Wiltshire in both January and February 2021 down to -4C in Salisbury to the east. Coastal redwood (Sequoia sempervirens) is frost tender when young becoming hardier but frost hollows should be avoided - Sequioadendrum giganteum likes similar conditions but can withstand a more acid soil and drier conditions. Western Red Cedar (Thuya plicate) also prefers sheltered  locations and not frosty sites. Probably most likely frost damage.

Edited July 25, 2021 by Vedhoggar