If more usual Tory voters switched to one of the Brexit parties than usual Labour voters, Labour might become the largest party, but I'd have doubts it'd be anything more than a hung parliament / coalition. And an excellent reason for May not to call another disastrous GE. Not even she could be contemplating that.
Worst Tory PM since Heath ? Maybe, but they're both competing for the title with Thatcher, mind. That said, her failures are clearly intentional and repetitive.
Trouble is, parliament (including May) is more keen on thwarting Brexit than in the state of their own parties. They care less about hemorrhaging votes than they do about ignoring the public and getting their own way.