The current ban on no household mixing has some exemptions (in all Tiers), including business meetings. This grey area was confirmed on 21/10 by Oliver Dowden. Obviously some of these meetings are with people outside your own office, at which point that can be in any covid-secure venue indoors or outdoors.
We will be allowing business lunches and dinners for up to 6 people. As we have already experienced with things like household mixing, we have some burden of responsibility but we are not the police and ultimately the responsibility for getting this right falls to the individuals. Therefore beyond reminding people of the rules as we do currently for households that is the limit of our due diligence.