I'm not sure that is the case. In the cases you mention, I do not think that there is any evidence that the police deliberately and knowingly acted in excess of their powers, knowing that they would lose any later civil action. What there have been (and will continue to be) are cases where the police seek to establish the boundaries of a particular law - that can only be done by bringing cases and taking actions that could be covered by the law and seeing what the Courts make of them. Once the Courts have set the boundaries, the police would be on very dangerous ground if they deliberately act beyond them in future cases as not only would there be scope for civil damages (which officers try to avoid anyway as such losses have a detrimental affect on their careers and professional reputations) but there may even be individual prosecution or civil suit for malfeasance in public office, a very old common law offence which is making a comeback over the last few years and which is a very powerful "backstop" against out and out improper police activity.
(Boundaries are usually set by the police pushing them out by the way, as there are very few situations in which anyone would have sufficient interest, motivation or legal standing to take a case arguing that police are interpreting their powers too narrowly!! So the fact that in these cases the police are always pushing the boundaries should not necessarily be taken as demonstrating overly robust policing, it's just that we never hear of cases in which they have been too weak as they simply don't exist, something which we should always remember when we hear case after case from the bad guys, going on about how badly they have been treated and we forget that there would very, very rarely be any case by a victim going on about how badly they have been treated!!)