But how would you enforce only downloading the relevant data? How do you get only the data you think is relevant before looking at the data? It could be the police couldn't agree that they wouldn't download more than they need because they simply do not have the expertise to be able to not download it all, and then process it afterwards. At the very least, someone would probably have to look at huge chunks of the data. And depending on how UK courts work, they may be required to get everything.

In the US, you would probably still have to get it all, but the prosecutor would likely be able to limit what was turned over to the defense. And if the defense wanted more, they might be able to get a neutral third-party to look at the data and decide whether it should be turned over (or the judge might do this). I don't know if the UK has something similar where the prosecutor can avoid turning all data over to the defense.

It might be that there is no good way to handle this type of case.