From the UK chief medical officers:

For both vaccines, data provided to MHRA demonstrate that whilst efficacy is optimised when a second dose is administered both offer considerable protection after a single dose, at least in the short term. ...

For both vaccines the second dose completes the course and is likely to be important for longer term protection.

The JCVI has subsequently recommended that as many people on the JCVI priority list as possible should sequentially be offered a first vaccine dose as the initial priority. They have advised that the second dose of the Pfizer-BioNTech vaccine may be given between 3 to 12 weeks ..

following the first dose, and that the second dose of the AstraZeneca (Oxford) vaccine may be given between 4 to 12 weeks following the first dose. The clinical risk priority order for deployment of the vaccines remains unchanged and applies to both vaccines. ...

Both are very effective vaccines.

The four UK Chief Medical Officers agree with JCVI that at this stage of the pandemic prioritising the first doses of vaccine for as many people as possible on the priority list will protect the greatest number of at risk people overall in the shortest possible time...

and will have the greatest impact on reducing mortality, severe disease and hospitalisations and in protecting the NHS and equivalent health services.

Operationally this will mean that second doses of both vaccines will be administered towards the end of the recommended vaccine dosing schedule of 12 weeks. This will maximise the number of people getting vaccine and therefore receiving protection in the next 12 weeks.

New on pregnancy: Vaccination with either vaccine in pregnancy should be considered where the risk of exposure SARS-CoV2 infection is high and cannot be avoided, or where the woman has underlying conditions that place her at very high risk of serious complications of Covid-19.