I'm not sure how the law works in the UK on a day to day basis, but normally they can see them, but they have no right to assume they are anything other than reptile or aquarium lights, until proven otherwise, usually with a security breach, a 'ratting out', or odor leak. Otherwise, they'd be breaking down a lot more pet enthusiasts doors
I've never had anyone knocking for either of my uses, for animals or growing, I've lived all over, and have used
thousands of watts in both residential and industrial property with choppers flying by all the while. I lived near an airport that the police took off from for more than two years, at one point. The main difference between me and people who get nabbed, is that I keep my odor in check, and I remember the old saying 'loose lips, sink ships'. If you keep your front perfect, they've got nothing to use on you.
If they pick up the heat, then walk by your flat and
smell a strong odor of growing cannabis however, you're done for.
There are materials you can use for blocking IR, as well as reducing heats capacity to escape and warm your room and roof, in suspicious patterns. Google is your friend
A single heat vent means nothing to them, you can install it like you'd install a drier vent. A pattern of lights is a bit more noticeable.