I hold a non-eu passport which is expiring and needs renewing. To renew it will take 4 weeks. I was planning to travel to London from Brussels via Eurostar whilst i await my new passport. I hold both a Belgian ID as well as a UK biometric card. Will travelling without my passport pose a problem for me at border control?
You state that you have a non eu passport, but that you have a Belgium ID card.
The uk requires non eu citizens to travel to the uk with a valid passport as detailed on the uk govt website.
If you hold dual nationality (and that is why you have a Belgium ID card) then yes you could travel.
But if not then no, you cannot travel to the uk, you need you passport to be renewed first.
I don't know about the UK biometric card, but regarding your Belgian ID card, it depends what kind of Belgian ID card you have. Although they all look very similar, there are differences, if it is for example an E+ card (proof of long stay) it is of no use outside Belgium. If it is an official identity card you are okay to travel with it. So it depends on what is described as the type of document.
If you're a Belgian citizen (i.e. a dual national) with a Belgian ID card, that's the only thing you should use at border control. It proves you're an EEA national, and they can enter on a valid ID card.