Common misconception, such as the one where people believe they can tell the police to get a warrant to search their car/look under the bonnet. Want a good hard and fast rule? Police can do almost anything they want, if you refuse, they will call on their special powers that entails all they require is "reasonable suspicion" or "reasonable cause" and you're stuffed anyway. If they do something wrong (or you think they did) take it to court (call a lawyer) the judge will decide and if the misconduct from the police outweighs the offence. You win.
However, for things such as searching your car or for anything actually (if you know you're guilty) you may as well make it hard as you can. Concenting only makes things easier as a concented search or admission of guilt makes it easy on the polices part. If they peform the search or whatever else through their own power, it may be deemed unreasonable and you might get the evidence excluded....chances are not good but if your guilty, deny, deny, deny, within reason.
note: This is not legal advice and should not be taken or acted upon as though it is. Consult a legal professional for your own legal advice before acting on any matter.
