Apparently they backed away from it anyway. Or it was a misunderstanding of what they said. I just checked again and NHS England have said it's extremely unlikely vaccines will be mixed.
"Dr Mary Ramsay, the head of Immunisations at Public Health England, said: “We do not recommend mixing the
Covid-19 vaccines – if your first dose is the Pfizer vaccine you should not be given the AstraZeneca vaccine for your second dose and vice versa.
“There may be extremely rare occasions where the same vaccine is not available, or where it is not known what vaccine the patient received.
“Every effort should be made to give them the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all.”"