Labour has actually been able to gain a large boost over the last week. Why? It has little to do with the terrorist attack, and, in fact, had the attack not happened, Labour might be leading in the polls.
The answer as to why is quite simple: The dementia tax. In short, it would require you to sell your house to receive care. That's quite a simplification and not exactly true, but that's most certainly how a large portion of the electorate understood it.
The reason this resonates is that it is similar in many ways to concerns under Cameron, in particular, suicides of disabled people.
In short: the Tories are seen as hurting the most vulnerable and just not plain caring about the damage they do. This does not go over well with moderate voters. May was seen as a possible change to that, but the whole dementia tax debacle has shown that this may not be the case.
As such, Labour is up.