The only chance Labour have in my eyes is to play the NHS card wth tax sweeteners in the biggest possible ways.
Flipping Brexit stance seriously risks further alienating their core (working and lower classes) and past voters, who are already disillusioned as it is. The elderly and lower classes swung the Brexit vote so the only way to re-engage them is to strike their core values in daily life (NHS, tax relief, wages and benefits).
I'm not sure there is enough time nor sufficient proof that in playing a flippant Brexit card in that manner will be capturing key demographics that will turn out (ie young and old) and swing the necessary margins.
Yet most people care about day to day services, healthcare and welfare. If Labours core messages can drum those key elements in early and loud enough, there is a feint chance. Sadly, there has been little evidence to suggest its likely either.
Reality is, this is a Labour whipping before our eyes as the Tories strengthen their positions, against most logic, due to the inherent ineptitude of almost every party around them.