I don't recall seeing an official protocol for this, but my understanding is that you can't further extend beyond the six months.
In normal circumstances (anyone remember those?), failing to meet your recertification window would land you with an automatic major NC against clause 1.1.10, but I would not expect this to be enforced, based on the extenuating circumstances allowance in the standard - see 2.8.2 in Annex on page 73 in part III of the standard.
Depending on your specific circumstances you may want to look into the new "blended" audits (see links below) as this would start the process moving but also buy you a bit more time to arrange the site-based component of the audit. I'm not sure how many certification bodies are actually offering these at present though, so you'll want to talk to yours if this is a potential consideration for you. In fact whatever route you are looking at, I'd get in touch with your certification body ASAP if you are at risk of missing the audit window - you should at least be able to start the process of getting agreement that you will effectively have a "justifiable" expiry of the certificate.
https://www.ifsqn.co...s-and-brcgs080/
https://www.brcgs.co...ting-using-ict/