5.1.5 Terminal design
When the diameter of a flue is 170 mm or less, a terminal conforming to BS 715, BS 1289 or BS 7435-1 and -2 shall be fitted.
Where a proprietary terminal is not used, the total free area of outlet openings on the termination shall be at least twice the cross-sectional area of the flue. Outlet openings shall be such that they will admit a 6 mm diameter ball but not a 16 mm diameter ball. The openings shall either be uniformly distributed around the termination or arranged at two opposite faces.
The free cross-section area of the terminal shall be appropriate for the appliance installed to the chimney.
That is from an old copy of BS 5440:1 (2000). Feel free to read on here!
As far as I know the 170mm is the same but check it yourself. It was the same in 2017 as I recall.
I read and understand the word 'shall' to mean 'you must and you will'!
As
@cjg said, I would install one as it will prevent the birds falling in. I have installed them on commercial flues a lot bigger than 170mm for the same reason. When the boilers/fires are off it is a nice place to perch and warm your bum if you're a bird. Once that blast of CO2 and steam hits you it's good night Vienna!
As far as I can tell from experience they are still alive but unconscious when they fall in. Fitting one would be a bird friendly thing to do.