Turn it off then if you want to - but that may not be the appropriate thing to do. How long you going to spend turning the mains off, before you spend 1 minute getting into the loft, to find the F&E is fed off a raised CWSC, then you sheepishly realise you cocked up.
You're there for an emergency, not to bring the house up to all the latest regulations.
He's going to know how to turn the water to the CH off, he's been fitting a rad. Use some intelligence and ask him. You're inventing problems to try to find something which a plumber knows about but I don't; you should because I'm not one, eventually, but so far it's very simple stuff and you're not doing too well.
Barge in there with your holier than the planet knowledge, drain the system rectifying everything you fancy on the way.
Drain the system like a proper diyer.
Build up that £600 bill.
Do up the nut on the rad valve spindle to actually stop the leak.
Then find he's got a Primatic and you have to restore the bubble. Epic fail Yeee ha!