It is important to consider the limitations of what people in uniform can do, if for no other reason than to determine whether or not any actions they have performed were legal and moral above all else. Security guards might not be the first people that end up popping into your head when you think of individuals in uniform that abuse their power, with that dubious honor usually going to police officers and the like, they are regardless people that have a fair amount of control over their surrounding environments when they are on duty so it can be fruitful to understand where the extent of their powers like once all has been said and is now out of the way.
A residential security team in London is usually entitled to use a low level of force if they are asked to do so by their employers. That means that they can grab someone to take them out of a building, and they can even become a bit more aggressive as long as this is required by the situation that they are involved in. A security guard is usually not going to be able to punch you, but if you were to try to assault them then them punching you would seem more like self defense and would therefore be more justified than might have been the case otherwise.
Security guards can’t physically hit people until or unless they have been threatened with violence themselves. In those situations they usually rely on batons or pepper spray, though, so they mostly avoid using their fists. Punching someone can cause injury so they generally don’t do that in most kinds of scenarios.