Yes, cops abuse their power all the time, because they know they have no accountability. The police help their friends get ahead. They protect their friends when the friends commit crimes. Those in power get away with things. Those without power live in fear. This is not new, but it is a sad thing for society.
Yes, police officers often abuse their power. Since police officers are the keepers of law and order, there is nobody above the police to oversee their actions if they are not being watched by their supervisors. This makes it likely that cops will abuse their power when they are confronted with tense situations.
In the case mentioned, police closed the pub due to a heavily attended football game the following day. The United Kingdom soccer fans have a history of extreme reactions following matches and the police were acting in the best interest of the pub ad the community at large. In other cases where Glasgow police have closed pubs, it was because of drug activity or fighting, so to claim that the police are abusing power is unfounded. Their tactics may not always be the best choice, but there is no proof to back up such a claim in this instance.
Yes, sometimes police do abuse their authority. If you mean Chequers, the pub posted that they were told to shut down by police in advance of a New Year's celebration. However, police from the Glasgow division released a statement to the contrary. If you mean Club Reina, it certainly seems that police were within their authority due to noise complaints and the presence of illegal drugs.