Can you say "no, you have no reasonable suspicion, therefore I have no need to undertake a breath test"?
The short answer, for all practical purposes, is No.
In law, if a police constable doesn't "reasonably suspect" you have alcohol in your body, then he isn't entitled to require you to take a preliminary breath-test and, accordingly, your refusal would be lawful and you'd have a good defence when prosecuted for refusing.
That's the theory.
Now ask yourself what the chances are, when the case gets to the local magistrates court, of the officer not saying, for example, that your 'demeanour' led him to suspect you had alcohol in your body. Or that he believed he could smell drink on your breath? Or that he thought your speech was slurred?
Bear in mind that he doesn't have to reasonably suspect that you have alcohol in your body over the prescribed limit - just some alcohol in your body.
Do the police even have to tell you what the "reasonable suspicion" is?
The policeman has to tell you he suspects you have alcohol in your body and that is why he is requiring you to take a breath-test. He does not have to tell you the reason(s) for his suspicion.
(He'd have to give his reasons if subsequently challenged in court, but see my comments above.)
In summary:
If required to take a breath-test at the scene, or to provide a specimen (breath, blood or urine) subsequently at a police station, you should do so - however unreasonable/unjustified you think it is.
If you refuse and are prosecuted, the likelihood of a court finding that you were legally entitled to refuse is extremely remote.
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I'll be offline for the next two weeks.
Tudor Owen