Don't forget that Labour (and everyone else) can say that May is breaking promises, getting it wrong, etc, etc, with emperical evidence. Whilst the Tories can at best say that Corbyn will break promises, get it wrong, etc, etc, merely speculating . It's the 'away advantage' to national political campaigning, to counteract the home advantage of not (hopefully) having annoyed/deported/allowed to die off a significant proportion of those who gave you the win the last time.
Everyone also does the whole "..and, what I say to you is, that in the fullness of time, at the appropriate juncture, when all conditions are right and we have our full and proper mandate, then we shall ensure that our export of floggletoggles and humgrummits to the Republic of Potarneyland properly balances our trade deficit, reduces our debt and increases employment and the standard of living for all working families in the inner citiy supurb of Ballykissangel!" thing. And stating what happened in the past (especially that annoying phrase "...that we inherited from the last government..." which I heard just a few days ago, despite that actually refering to three governments, and a global crisis, ago...), which has as much or as little relevence as you care to personally give it, even assuming you agree with the 'facts' being given.