If you have a hart attack you don’t lose your licence you can drive on your doctors say so normally after six weeks it is then up to the DVLA to arrange a test up till that point you can drive ok.
The DVLA will Evan give you a letter to show your boss that you can drive. I know this because I had a triple hart bypass in June 2012 went back to work in October 2012 and didn’t take a tread mill test till may 2013(passed)
If you cannot pass the tread mill test the DVLA will remove you hgv from you and you can retake the test at latter date on appeal
Hope this helps