As part of my world's planetary history, an asteroid impact has delivered a sizable quantity of a previously unknown metal to the planet Earth. By sizable I mean a deposit large enough to sustain a mining programme for several years, possibly decades. This asteroid will land somewhere off the west coast of mainland England, presumably destroying Ireland and leaving a crater many miles across. Obviously this will be an extinction-level event.
The tales I intend to tell will document a technological society living on this altered Earth. My problem is how long would the Earth be rendered "uninhabitable" (i.e. impact winter making life on the surface very difficult, or even impossible) by such an impact? Are we talking thousands, tens of thousands, or even millions of years? And what sort of knock-on effect would that timescale have on a human population living underground?
Would it make more sense to have several smaller impacts or one massive impact? My plans involve one rather large deposit in a single place, but if this doesn't make much scientific sense then I have contingencies in place for that.
EDIT I'm now leaning toward a large-scale impact off the west coast of England, and two - or possibly three - smaller pieces of the same asteroid landing further west along its orbital track, assuming the asteroid came down in an east-to-west direction. So possibly one landing mid-Atlantic, another on either the east coast of America or the east coast of Canada, and a third in either Russia or China.