If you hadn't noticed, this is the electrics forum. Most people on here are electricians and as a result, won't be experts on water regs - regardless, I'll give your question a go
If your neighbour has access to the stop cock then common sense should tell you that he could physically turn off your water without telling you first, just as I could walk down my street and do the same if I was feeling particularly mischievous.
That isn't to say I wouldn't be commiting some offence, breaking some water regulation or breaking the conditions of use. I'm guessing you have some issue with your neighbour for you to ask this? The main stop-cock to my property is outside on the pavement, and it has never crossed my mind that someone might maliciously deprive me of water...
When I was a student, shortly before Christmas our water went off, everyone else who we spoke to nearby still had theirs on. I noticed the old external stop cock was no longer in plumbed from the now capped pipe coming from the ground (presumably a problem with it at somepoint) but the incoming pipework came from nextdoors house. Nextdoor were also students and had headed home for Christmas turning off the water before they went (in fear of burst pipes), fortunately they had the same landlord so it was easily put back on.
Doesn’t really help you other than pointing out it can and does happen. We had phoned Yorkshire Water when it first went off (before other neighbours were home) they sent a guy round, by then I had figured the likely cause and he basically said it was nothing to do with them but gave us some bottled water to be getting on with.