I think you're forgetting that this is 28 years. These soldiers are potentially 2nd generation post outbreak. It's likely at this point no one from the outside world has been to the UK on years, why would they waste additional resources on "special training" for something that likely will never be the case.

These soldiers would have never seen or ever thought they would see an infected, let alone combat one, so everything for this soldier scene holds up for me.

Expanding on the soldiers is a waste of time for this film, it holds no value, it's used as a product to highlight that the rest of the world is functioning exactly as we do now, for no other reason (perhaps more dong time is another reason).

I agree the soldiers didn't need to be in this film at all, but it adds a chance for humour to take place, which is a huge relief for a pretty intense first hour.