Being scruffy is equally insufficient on it's own as the basis for stop and search as is ethnicity.(a) is decidedly dodgy, but could be justified. I don't trust the police with this judgement, though. It's too open to abuse, generally speaking. I see it regularly where I live – people are harassed not for being black, but for being scruffy. People are told they can't sit and drink in certain places, so they just go elsewhere to do it, often outside my door, as it happens, as there are steps to sit on. All they want to do is sit and have a drink, though. I have never seen any trouble from them. If in doubt, leave people alone!
The issue of people sitting around drinking is something different. It is where a particular type of activity (street drinking or whatever) has been identified by local people as a problem, acting through the local authority and / or the police, and a Dispersal zone or other type of order has been put in place to deal with it. Here it is very much the activity which is targetted, not the characteristics of the individual.
I'm not sure how you can seperate (b) and (c) in some circumstances. If there is a robbery tonight, then two more tomorrow and three more over the next week, surely action taken on the basis of the analysis of them all together is actually likely to be based on stronger grounds than action taken later this evening on the basis of one?(c) is unjustified. Simple as that. The police – and the rest of us – need to find other ways to address such problems.
And could you please suggest some other ways in which the police could "find other ways" to deal with situations in which we know something is likely to happen but we don't know who is likely to do it? If someone rings Crimestoppers and says that, say, there is a major planned fight between Chelsea and Millwall supporters to happen at London Bridge Station on Saturday, with knives and other weapons to be used, what should the police be able to do? If they do not make use of some sort of preventative powers they will, by definition, have to wait until an actual fight starts which, if that big, will then result in serious injuries and possibly worse, major danger to ordinary citizens using the station and will likely be impossible to quell without significant injuries caused to and by police officers. Do you really think that that would be better than using preventative action to try and prevent or disrupt the event before it even starts?