Not sure where to ask this, but we've brought gold passes this year, we've never had passes before. How late can you reserve, are on the day reservations allowed?
Apart from Fireworks night, I've not reserved a visit to Alton Towers in advance since 2020. I've not been checked either. In fact, I've never had a reservation checked at any park, apart from a couple of times during Covid, soon after reopening.
I have reserved Towers on the day a few times, when I've made plans to meet others there. (But not always, I usually just go with no reservation.) I do have a platinum pass though, so that may make reserving on the day easier, as reservation numbers are limited for the lower tiers, whereas Platinum is supposed to be available, without restriction, any time tickets are available.
Also, I ALWAYS reserve the southern parks, because I wouldn't want to risk travelling all of the way down there, only to get turned away. (Still never been checked at any of them though!)
Long story short, you can definitely reserve all but the busiest days on the day, with a platinum pass. With a gold pass, I'm not sure, but I imagine that, if there's space, then you can reserve. There's no mechanism or rules specifically preventing it, just a tighter limit on the number of reservations available to the lower tiers. (And if you like living on the edge, and there's no reservations left, make the journey anyway, but don't blame me if your mileage varies, and you do get checked!)