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I unfortunately have a holiday forced on me (family etc).

I submitted Form AN and the HO emailed acknowledgement on the 18th Sept.

I know that I'm going to get a request for biometrics at some point soon - can I do this in advance at a post office, or does it have to come after the letter?

I'm away for ~12 days between 27th Sept -> 8th Oct.

No.

The Biometrics Letter contain a unique QR like field that needs to be scanned by the Post Office. You have 15 days from the date of the letter to have this done so you may be ok, otherwise you'll get a second letter giving 10 days, after that your application will be returned.

Thanks for your answer.

Do you think I should write to the HO? I sent an email saying I'd be unavailable for a bit, and also called them, but the person on the other end of the line didn't really seem to care to pass the information along, just told me not to travel...

edit: Judging by the timelines in other posts, I may well get a chance to do the biometric thingy before I leave (if the request arrives before 26th, seems like it doesn't usually take longer than a week to get the letter).

The response to your enquiries tells you that writing a letter likely won't help either.
You may have it by the time you return - get somebody to check your mail so you have pre-warning.

You might luck out and get the biometrics letter right before you leave for your holiday, but it's likely that it will come while you're away. You have 15 working days from the date of the letter, which means that even in the unlikely event the letter is issued today, you'll still have time to submit your biometrics once you get back from your holiday. Just plan on doing it the first day that you're back.

It came today I'll make a timeline post to help others gather evidence so they can hopefully have a go at guessing how to handle their own situations.

There are already numerous timeline threads. Kindly post in one relevant to your month of application.