The NHS website says (my bolding):
I rebooked my booster at 27 days after the positive PCR test but more than 28 days after symptoms (or sympton in my case) started. This is about a week before I am due to leave the country (assuming not still positive with a PCR). So I would still have time to find a walk-in booster clinic over Christmas if they went by the PCR date but would prefer the booked slot if possible.
I go by the date the PCR. Some people still won't get tested for various reasons (and my own PCRs are now held up in a PCR processing backlog), hence the other option. Particularly true of children it seems. Anyway I personally would send you away on day 27. Clinics are working over Christmas, we will be working on Christmas Day, Boxing Day and New Year's Day too, so you should be able to find a slot on day 28.