The idea with contact tracing is that you identify people someone who has tested positive has come into contact with. You then test all of these people, then if they test positive you interview them, and test people they came into contact with. And on and on.
You don't just stop with the first person and say quaranteen for however many days.
They need to put proper contact tracing in place. Hopefully this is just a stopgap while we are still in lockdown and a proper contact tracing regime will be introduced when we start to come out of it.
They need to get a shift on with the app, and make it mandatory on public transport. At least in London. You don't get through the barriers unless you can prove you aren't carrying the virus.