What vaccine?
I helped out at Wimbledon Centre Court yesterday and we pretty much denied anyone a walk-in second jab of AZ unless they were close to their 8 weeks (unless they had an urgent situation or flight booked for the next day or something). Maybe not worth people doing a trip unless it's in a convenient location!
That 21 days was in reference to Pfizer
As you say, Az/Oxford ideally shouldn't be given to anyone prior to 8 weeks except in exceptional circumstances.
I'm going to quote the section on dosage schedule from the Green Book for health professionals I posted earlier, just for the sake of clarity (with the source at the bottom). Ignore the dosage volumes obviously, those are irrelevant to us
Dosing and schedule
Pfizer BioNTech COVID-19 mRNA vaccine BNT162b2
The dose of Pfizer BioNTech COVID-19 vaccine is 30µg contained in 0.3ml of the diluted vaccine. After dilution each multidose vial can be used to deliver five or six doses of 0.3ml.
The vaccine should be administered in two doses, a minimum of 21 days apart.
AstraZeneca COVID-19 vaccine
The dose of AstraZeneca COVID-19 vaccine is 0.5ml.
Based on good evidence of higher clinical protection, JCVI currently recommend that, ideally, an eight week minimum interval should be observed for this vaccine. An interval of 28 days may be observed when rapid protection is required (for example for those about
to receive immunosuppressive treatment).
Moderna COVID-19 vaccine
The dose of Moderna COVID-19 vaccine is 0.5ml.
The vaccine should be administered in two doses, a minimum of 28 days apart
No need to spend any time reading it, but the source of this is here https://www.gov.uk/government/publications/covid-19-the-green-book-chapter-14a