Green Channel: "Nothing to Declare"
Red Channel: "Articles to Declare"
By going through the Green Channel, you are implicitly confirming that you have understood the applicable allowances for entering the UK and confirming that you have "nothing to declare", as it says on the signage. Going through the Green Channel, then getting caught with goods over the limit,
then saying "I want to pay the tax" is
not "being given opportunities to pay" - it's more like taking a gamble and losing. A gamble that I'm sure the vast majority of people take, and since they can't screen 100% of passengers, a gamble that a lot of people get away with. But when caught, one doesn't have much of a recourse.
By going through the Red Channel, you are acknowledging that you have goods that may be taxable and you are willing to pay the applicable taxes to import those goods into the UK. Thus, the Red Channel
is "ample opportunities to pay".
In the end, HM C&E have the right to extend
some flexibility, but it is
their right and their right only. If you go through the Red Channel, declare to the officer that you have x and y amount over the limit, but the officer still says "That's fine, just go ahead", that is HM C&E exercising their right to extend flexibility. As a traveller going through the "Nothing to Declare" lane, you don't have that flexibility and you have already given up the opportunity to declare the goods and pay the applicable tax.
At Heathrow at least, they have quite large signage with the duty-free limits clearly displayed on the baggage carousel. Maybe Aberdeen Airport is too small, but if in doubt, you
should have asked an officer - As with everything else, ignorance of the law and regulations is not a valid defence
PPS. I really don't mean this to have a go at you as such, but just trying to clarify the reasoning behind their actions. The daft regulations and stupidly low limits (including the £135 (now £500) limit on non-alcoholic/tobacco/perfume goods, purely based on total value) gets on my boobies too, but in the end, caveat emptor.