Cases are going up because of more testing revealing them, but the key thing to look out for is % of tests that are positive - it's a better reflection of actual spread than raw case numbers.
And you can't really just hope and pray that it'll be better than before just because cases have been rising slowly since July without an accompanying rise in deaths/cases requiring hospitalisation. Think of it this way: We know that case numbers are rising predominantly in younger demographics, whereas the first "wave" was more evenly distributed (and the deaths were more common in people aged 50+). Whatever we're doing in terms of behavioural changes, so far, seems to be working in terms of balancing the spread against the need to give us all some degree of normality under stress (lockdown has its own health costs). BUT, at the same time, there hasn't been any appreciable mutation detected in the virus - it doesn't seem to have changed much over the year. This is an important thing to track, because it does give us a sense of understanding whether a virus like this is actually more or less dangerous. The Spanish Flu is a good example of the danger here, as you say, but it really is just a crapshoot - it could go up, it could go down, it could stay the same. You kind of have to assume the worst-case beforehand, because it would be too late to react and do anything if you're wrong by the time it's apparent.
If we take all that together, the balance of risks becomes clearer. Even with widespread changes in social behaviour - yes, it's not perfect by any means, but people are still generally more cautious than "normal", and many habits that are dangerous are much reduced like eg large-scale commuting, large cultural events, large gatherings, etc - we're still seeing cases start to rise, and the nature of exponential growth is that things kick off faster shockingly quickly, counter-untuitively so. We've just sent kids back to school, which is the largest mixing of different family and social bubbles since the lockdown, and university students are also heading back. If you already have the disease endemic in many communities, you're not just restarting the spread, you're giving it rocket fuel. And, crucially, at a certain point the "younger demographics" which have been sucking up most of the new cases will become saturated, and overspill. We don't know when or how that will happen, but it'll happen, same as hospitals becoming full kicked off a wave of deaths in care homes as people were discharged back into the community. Plus, of course, there's the lag between infections and deaths, so you really do have to be proactive when you see large case rises and nip it in the bud rather than waiting and seeing, as happened here in March, when it was clear for a few weeks that the UK was tracking Italy's case/death ratio almost exactly yet took measures in response later along the same trendline, and tens of thousands died as a result.
Interestingly enough, I got a letter this week saying I'd been randomly picked by the ONS to take part in the nationwide study they're doing to figure out exactly how widespread infections and immunity are. A health worker's going to drop around every few weeks and give me and my partner tests, but since it's projects like it that are the only way we'll know for sure how widespread infections were in the first wave, and in any incoming second, I'm pretty stoked to help out with it.
Whatever happens, the key thing is that lockdowns are really a policy failure whenever they happen. The whole point of what we've been doing is that it buys time to get the health infrastructure in place to suppress infections whenever they bubble up, so that even if you can never fully eliminate it you can at least get things back to as-near-as-normal ASAP. Having to go back into a second lockdown is just... Well, it's many things, and doesn't really do my rock-bottom opinion of this government any favours, but one thing it does lend credence to is the idea that we are still paying for early mistakes in, when a lockdown became clearly necessary, going for it as hard and comprehensively as possible. But I don't know, what do you guys all think? If someone came to me back in March and said "you're going to have to live in full quarantine, no leaving the house, not even for shopping, Wuhan-style, for 6-8 weeks, but afterwards it'll basically be back to normal", I'd take it in a shot.