the people who would opt in aren't likely to be the problem. The problem with your pickpocket example is that you lose something when someone picks your pocket, but you lose nothing when someone checks to see what ports are open.

In fact, that's something that's already happening all the time anyway. The only difference is that in this case the person checking for your failures to secure your devices will notify you of the problem instead of exploiting your devices like everyone else will (assuming that they haven't already).

This should not only help people secure their devices, but it should also make the internet a better place for everybody.

If this service causes a bunch of crashes (somehow) or they end up DoSing someone they should be responsible for the harm that they cause, but since these scans are no different that what criminals are already doing every day I don't imagine it'll be a huge problem unless they really screw something up.

I'd also guess that the costs in both time and money spent on the traffic generated by DDoS attacks, malware infections, and phishing sites are much much greater than the costs for 'security guys' to review logs, safely automate scans, and notify webhosts of problems. This is a sensible measure that should save massive amounts of time and money for people all around the globe and make the internet better for UK citizens in the process.