The UK watchdog has banned adverts by cryptocurrency company Luno that said it was "time to buy" bitcoin from reappearing in their current form, calling them misleading and irresponsible.Luno - which is owned by Grayscale-owner Digital Currency Group - had put up ads across London's transport network which said: "If you're seeing Bitcoin on the Underground, it's time to buy." A similar message appeared on buses.But the UK's Advertising Standards Agency on Wednesday said it had upheld complaints about the adverts, and ruled they "must not appear again in the form complained about."It is the