LONDON, July 19 (Reuters) - Britain on Monday announced new exemptions to its rules on self-isolation for those exposed to COVID-19, saying that workers in critical roles could be allowed to continue working instead of quarantining for 10 days. "We recognise there are some very specific circumstances where there would be a serious risk of harm to public welfare if people in critical roles are unable to go to their workplace - like air traffic controllers or train signallers," vaccine minister Nadhim Zahawi told parliament. "So people in those kinds of roles, who have received two vaccinations, plus two weeks beyond the second vaccine, will not need to self isolate for those critical tasks they will, however, have to continue to self isolate at all other times." (This story has been refiled to add dropped letter in minister"s name)