Chris Whitty didn't say there won't be a vaccine for 15 months, he said we should plan for this winter on the basis of what we currently have (i.e. no vaccine), and if one is approved in time, we will be in a better position.
The clip with Prof Adrian Hill in the article that @Ellsbells12 linked to is well worth a listen.
I reckon the rollout order will be:-
1)Frontline NHS and care staff, possibly other emergency services
2) Over 70's plus younger people with medical conditions which make them vulnerable
3) Over 60's
4) Over 50's
et cetera down the age scale.
The first trials only involved healthy adults aged 18-55, but later they were recruiting older adults and people with some health conditions. So by the time they are able to report efficacy with the main group, they should also have safety data for the other groups, if not efficacy data. That may allow the regulator to approve it for older people, on the basis that it is safe.
Let's say in adults 18-55 it is 95% effective, but in over 70's it's only 50%. That is still worth having, so long as the risk from the vaccine itself is sufficiently low.