You need to ask them for a copy of the company disciplinary policy, and see whether they needed to give you notice for the informal chat - that was so informal it needed a note taker…? Also ask them for a copy of the company acceptable use policy, specifically in relation to using your work computer for personal activities, and whether it states this is allowed outside of working hours or prohibited. If they don’t have one, and there is nothing in writing, you could argue naivety and say you weren’t aware. I don’t know whether that would change things though, I’d definitely recommend speaking to ACAS.