Having done this sort of thing many times I can say that for what 'normal' people do, like bringing in 400 cigs or 2L of rum things are pretty simple. Beyond that there are worms ahoy. There are also differences between personally trasnsported goods (you) and stuff imported via post and via third parties...
But unless stuff is high value, illegal or otherwise taking the p, uk customs is generally quite lenient and I can recall at least two times I got pulled going through the green channel when it was a fair cop, but they obviously didn't think the paperwork was worth the effort. (10 years ago though). They target passengers based on luggage scans, so if you have litres of the good stuff there is a high chance you will get pulled even if it's a low chance to get nobbled.
They are really looking for cig/booze excise fraud and narcotics. Outwith that (which syrup comes under) you are prob ok. But note that the same places that make syrup also make Cannabis products which might attract attention.
My worst fail in this regard was poker chips. I got stiffed for 100% tariff on chips imported from Las Vegas, on the shipping as well as the retail. It wasn't even a lot of money (at least to start with).