It's a while since I've looked at this particular area in any detail (I don't work with bakery), but my current reading of the situation is that your colleagues are probably right, and I think you possibly should be packing/declaring to average weight.
My recollection is that some of the previous exemptions in the UK changed following 1169/2011 coming into force, so an amended list was published as part of a number of "corrections" to UK regs as a result of that regulation, in the form of the Weights & Measures (Food) (Amendment) Regulations 2014. Schedule 1 of this regulation details the exemptions and this does still include e.g. your biscuit example, but to me it does not look like a brownie would fall into any of the listed categories.
It could be worth reaching out to the British Society of Baking (or possibly the Craft Bakers Association if more appropriate for the size of your business) as they may be able to give a more solid steer on this, unless another forum member can assist.
In general I wouldn't be too concerned about reaching out to Trading Standards on this either - you are trying to improve your business processes, and trying to ensure you're fully compliant with relevant regs, both of which are aims that they are generally very supportive of, IME. The very worse case I'd expect from this type of conversation would be that if the packs should be weight marked then they may ask you to refrain from producing more until corrected.
But I would have a quick look over the rest of your labels before doing so, as probably easier to do this in on go as a "get them all fixed" exercise, or at least be aware of other things that may get picked up if they start looking, so you can respond positively about it already being on your agenda to address