FWIW, under both English and Scots law evidence gathered by such means cannot be used in a court of law; though, certainly, it can be used by police (and often has been in NI to great success) as a means to observe crimes being planned (where you don't have evidence to prosecute, typically for conspiracy to murder in the NI case) and then have police ready to act to thwart the crime (thus the number of IRA bombers caught on their bomb run, at police checkpoints set up at the last minute).
But can such evidence be used to establish the grounds for a warrant?
That is a common trick in the USA. Data obtained in ways that would not be admissible in court can be used to get a warrant, and then data collected under the warrant can be used in court.
Grounds for a warrant is a matter for a court, and as it cannot be presented in court, no.
The typical behaviour in the days of IRA activity was that an attack being planned would be discovered through espionage (be it wiretaps, double agents, etc.) but no action would be taken (though several crimes have already been committed: membership of a terrorist organisation, conspiracy to murder, and often various other more minor offences) until they had the bomb with them, on their way to the attack, as until that point they didn't have evidence they could use in court. Typically a road block was set up, quickly (the bomb was normally transported by two cars, about a mile apart, the first one there to spot any police to abort it, so the roadblock could only be set up after this had past), by police and army. Obviously this relies on the people wanting to get out alive (I.e., not willing to blow it up with themselves in hope of getting arresting officer), but while they'd often put up a fight, they normally knew when it was over.
