But only they are allowed to beat us senseless without fear of getting in trouble for it.
But they are
not allowed to "beat us senseless without fear of getting in trouble for it". They mainly have ONLY the same rights to use force as anyone else: in self-defence or defence of another; to prevent crime or to make a lawful arrest. The owners of private property have an additional right that the police
don't have (to eject a trespasser). (There are a few situations, associated with the carrying out of authorised actions that would only apply to the police, such as the taking of fingerprints after arrest and various other powers under PACE, where the police have powers not available to the general public, but they are not often the ones being used in the
vast majority of cases). In every case the force used, as for the public, must be "reasonable and necessary".
If they use any force when they have no legal power to they are investigated, charged and prosecuted. If they use excessive force when they have a legal power to use some, likewise.
The only reason that you perceive that they are somehow not subject to the law is that the vast majority of cases (but by no means all) in which force is used (especially in the preventing of crime or making of arrests) it is the police who are involved. And the main reason that there are few convictions is that they almost always have a lawful right to use
some force and the amount used is rarely so excessive as to clearly merit conviction in an area where the law requires judgement to be made on the basis of what the person using the force honestly believed at the time (even if it turns out to be wrong) and acknowledges that it is not possible to weigh to a nicety the amount of force being used. Thus, as the vast majority of officers are usually trying their best to do their job properly and lawfully in difficult and often confused circumstances there are not going to be many convictions.
You are looking at an outcome (relatively few (but by no means no) convictions of the police for using excessive force) and concluding that it means one thing (the police are above the law) when in fact exactly the same outcome would arise from another thing (the police usually acting within the law).
(in moon23's example, by the way, there was certainly power to use some force (she was obstructing them in making an arrest) but whether or not being pepper-sprayed / batoned was "reasonable and necessary" is less clear.)