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I wanted to apply for a particular postdoc in the UK that has a start date in September. It is a great opportunity but I would need to delay the start date as I am currently in the US and it would not be possible for me to move before then.

How common is it to delay the start of postdocs? Is it worth applying even with the knowledge that I wouldn't be able to start by the advertised date?

It is very common to negotiate the start date. All of the postdoctoral researchers I hired or was involved in recruiting came at different times than advertised. Most funding bodies allow you to defer funding for a few months. For instance, EPSRC allows up to 3 months delay for start of projects. Also, most funders allow a so-called no-cost extension, meaning you can extend the end of the project without losing the money.

Just apply and in your cover letter say that you would be available in October, for instance.

Source: I am faculty (engineering) at a UK university and have been involved in recruiting 5 postdoctoral researchers.

Not especially common, no. But you have to take it up with them to see if it is possible. No one else can answer. It depends on field, generally, and on the particular needs of the institution. In some fields it wouldn't be especially disruptive to have you come in late, provided no teaching is involved. But in some lab sciences (for example) there might be a firm need for a person in place by a certain date.

The agency funding the postdoc might also have firm dates in mind, after which funding ends.

But it would be a mistake to apply without exploring it first with them. If you get pretty far along before you find out you are actually not eligible, then you've wasted everyone's time.

I would definitely ask them if that could work. Sometimes the timelines on funding are restricted, but often it's not set in stone and there is some flexibility. Be honest from the outset about what your existing commitments are though - I would definitely suggest getting in touch before doing the formal application.

My current postdoc position (also in the UK) wanted someone to start straight away, but I was in another post and was really keen to see that project through, so they were willing to delay for 7 months for me. I think they actually respected that I was loyal to my existing role, even though it was ending soon. For postdoc positions, I think getting the right candidate is often more important than getting a candidate ASAP (if that's possible within the scope of the funding).