People can use a lot of electricity without generating a lot of heat in a room that looks like it would make a nice grow room. In other words, a very small percentage of people using a lot of electricity are growing pot. The percentage of people with a hot room and hydroponics garbage in the back dumpster that are growing pot is a lot higher.

I don't believe that helicopters are specifically sent out to look for hot roofs in the UK without a reasonable suspicion.

I believe that the power companies are more likely to be of use in detecting cannabis growers. I am sure that in some areas a mini arms race has gone on for decades with cannabis growers taking electricity from the 'other side' of the meter, so that nothing shows up on the bill. I am sure that electricity companies have also worked out how to compare electricity supplied versus electricity charged for in order to detect this, at least to street level, and, after that, the infra-red camera can come into play to detect the hot-roof, either by themselves or by the police.

Given a choice of stealing electricity for cannabis or bitcoins I would be surprised if the latter was more profitable. That said, bitcoin miners probably do arouse the suspicions of the authorities. Therefore it would be quite convenient to have a 'server farm' downstairs with a 'cannabis farm' in the loft. Therefore, when the authorities come round there is a plausible reason for the electricity usage.

Growers steal electricity to avoid detection by the power companies auditing their usage. Considering that a grow-op can produce hundreds of thousands of dollars, the power bill's just a blip on the radar.

They bypass the meters taking all sorts of Jerry rigged risks when doing so.