There is an argument that spoiling the ballot is still more effective than not voting at all, so I’d recommend that if there’s definitely no one you want to vote for!
Well, yes. If a party does prioritise electoral reform, there may be other reasons why they strongly don't want to vote for that party. If someone is determined that they can't for any of the parties, then spoiling the ballot is at least better than not voting at all. There is no way i'd vote for a party like UKIP for example, if they happened to also support electoral reform.
You are right in suggesting that in the right conditions a seemingly secure majority can be overturned. I can only speak for Canterbury (I live yards from the constituency boundary) but I suspect the Conservatives were complacent (which may have been true of Labour in Sedgefield). Their candidate had been there for ages and had lost touch; Labour got the student vote out (2 universities and an offshoot in a small city) and fielded a likeable candidate who almost doubled the Labour vote.
With Sedgefield there's the assumption that the entire constituency is just monolithically Labour colliery villages. There is an element of that, but Newton Aycliffe is a socially mixed new town (and has some well off estates towards the outskirts), Sedgefield is a very pleasant market town which nearly always sends at least one Conservative councillor to County Hall, and the villages around Darlington are downright affluent. There has also been an increase in the number of commuters living in the area, given how handy it is for Durham, Teesside, Sunderland, and even Newcastle, while Labour's core vote there has rapidly fallen out of love with the party. Frankly, the fact it was ever properly safely Labour is the surprising part.
It certainly gave it a symbolic value, but the seat Blair won in 1983 no longer exists in 2019. The social changes there have been very dramatic, indeed that could be said for both of the other two Conservative gains in rural County Durham.
Doubtless, with closer inspection we would find there are a lot more potentially marginal constituencies. Quite possibly the breaching of the 'Red Wall' has more to do with Labour neglect of its traditional support than a 'love-in' with Boris. I notice that in 2017 and 2019 several old-guard Labour MPs lost their seat. There are still plenty of seats where the colour of the rosette defines the winner, but, I suspect, fewer, not necessarily because voters have switched, but because some have stayed at home and others, previously non-voters, have turned out. Every reason to give the underdog a chance and not waste the vote.
Of course, the New Labour of Tony Blair way well have appealed to voters in the commuter belt, described above, but they will have been absolutely turned off by Momentum.
