Years ago, I had to sign a similar opt-out. My job was 9-5.30, but if we ended up staying late, overtime pay didn't kick in till 6pm onwards. And I did earn a fair bit of overtime when we were coming up to project deadlines.
Maybe I remember signing that that opt-out because it was the first time I'd had to do so? Because I can't remember having to sign one for jobs since then.
I think the thing to look out for, is how many hours is the standard working week, what's the start time, what's the finish time, and how long is lunch and other breaks?
I got caught out in my current job, by assuming lunch hour when he was asking me, verbally, if £x amount was okay for the salary. But then the actual offer letter was emailed to me and it turned out lunch was only 30 minutes. I pointed out that meant the hourly rate was lower than expected.
That's one thing I've noticed, ie sometimes you get a call with a verbal offer, 'We'd like to offer you the job, the salary will be X, is that okay?' If you're put on the spot like that, I think the tendency is that you might be too excited/relieved to have a job offer that you'll just accept, because you don't want to start on on the wrong foot by coming across as awkward. But it's standard to negotiate, ask for more.
But moreover it's important to ask for time to consider the offer and then go through it with a fine-tooth comb.
My current contract says something like working additional hours might be expected or needed, depending on the needs of the business, and overtime isn't payable unless agreed in advance.
I did end up working some (unpaid) overtime to meet deadlines. And when I went back and checked my contract there was nothing in it about TOIL.
I ended up getting quite frustrated and annoyed and stressed out in month four, because reasons. And ended up emailing my boss and saying I was taking some TOIL, because otherwise - as I wasn't being paid for overtime - all the overtime I'd done meant there was a risk my effective hourly rate fell below the National Living Wage, which would be illegal.
In hindsight, I think it's now a red flag. Especially when working for an SME.
You could try saying you've never had to sign anything like that before and can they explain to you how it works in practice, can they eg let you know how much overtime the previous incumbent did, and if the overtime was paid, or if they accrued and took TOIL?
I think if you're being asked to sign a document agreeing to work overtime, it's reasonable to ask how that overtime will be remunerated/what the TOIL policy is.