I’ve travelled on hundreds and hundreds of trains in the UK. On busy services, unless you’re getting on at the start, if you don’t have a specific seat reserved, you will be standing. This is normal. I don’t have a source for that claim, I just have many years of experience.

There’s a plausibility gap on capping ticket sales for trains. Why on earth would they stop selling “anytime” tickets? They’re really expensive and a train with plenty of people standing costs the company no more but earns them a great deal.

What’s unusual here is that this looks like it came with an “advance” ticket, which is cheaper, limited in number, only available in advance, and is required to come with a reserved seat, but they’ve clearly oversold even them.