I don't think this is the nightmare it's being made out to be, and tactically, I think it actually puts labour in a better position for the next GE, basically, it's giving the tories more rope to hang themselves, in a weird way. It's a short term loss for a long term gain.
In reality, the Hartlepool seat was lost years ago (like many others), mainly due to Brexit and "get Brexit done et", it just wasn't realised in the numbers with the split Brexit/ far-right vote. Everyone that left before, has no reason to come back, as the reasons they left were for right-wing/ Brexit reasons and we're yet to see the main problems of this pan out yet.
The way I see it is Labour would be wrong to be seen as being "opposition" to the government now, when we're fighting a pandemic (even though the tories have done a superbly $hit job), loads of the easily susceptible new tories would not likely "come back" due to this kind of opposition, not now anyway. Don't forget "this lot" are all about "get behind and support brexit, its failings are because remainers are negative", they're a weird bunch, they think believing in something is more important that than the reality of it.
We've also pretty much nearly completed being the first major nation in the world, to have an unreal vaccination program. Labour could have anyone in charge and any tactics, and they would not stand a chance against this, it's a certain loss, and cannot be any other way, they would be better of saying "fair play Tories" or direct the positive on the NHS, despite how much it's been hammered etc. It won't get them any votes and may lose some of the far left, but it won't pi$$ off the Brexit/ middle/ right lot any more, and they literally cannot win without some of their votes (unfortunately). Losing the far left is irrelevant, for now.
So the way I see it is, let the vaccine and pandemic dust settle (little can be done anyway, by opposition), and watch how the cost of this and the cost of Brexit start to bite, then in the 1-2 year run up to the GE, start to go after the Tories hard. At the minute "brexiteers" are still enjoying their "win", they won't come down from this jizz fest for a year or so yet.
Basically, the way I see it is, if you take the "get brexit done" false positive, actual vaccine positive away (which will be short-lived and soon forgotten, especially as the EU is catching up fast), and lockdowns ending positive away, and remember there is no far-right competition for the tories then we're basically still in the same position as before. The thing is labour have not really tried to hit the tories and I think it's intentional, let them have their peak, and then go after them when the timing is right, and there is going to be a $hit load of ammo to come:
Covid austerity
Brexit austerity
Scottish independence, either they leave and we get weaker or they stay and labour can maybe win back some votes or work with them
Ireland problems
GDP growth deficit compared to the EU
150k covid deaths which is never going to look good
We will have the same % vaccinated as anywhere in the EU, come winter (so the vaccine is no longer a "win" for the tories, as it's no longer better then the EU)
Inquest into the handling of covid
Comparisons between the handling of covid in different countries once it's played out
Furlough ending/ jobs lost
Summer end of lockdown buzz finishes and winter of discontent starts
There's a lot of big problems coming there, and the tories will not look after the most worse off (a lot of whom, voted brexit), and no way can the tories maintain what they have now, through all that, then all labour need is some momentum and the get the other left parties tactically voting.
Even if labour gained some now, it would probably not make much difference anyway, it's a waste of a gain, if you're going to be in power during some exceptionally rough times (so could lose again).
Effectively it was going to be ****, our own idiot people seen to that, so just let it be **** under the tories and then you can point blame at them easier later.