We could always go to the bone yard and ask the cousins for a few Harriers...
If there had been enough personnel and budget to consider running another fleet of fast jets, it would have been better to spend them on retaining more squadrons of Tornado, which would have allowed the maintenance of a greater weight of effort than seen presently. The pre-2011 plan was to retire Tornado in 2025, not 2019, so there would have been plenty of airframes available with which to run 5+ squadrons (which, granted, might have brought the OSD forward a bit) which in turn would have reduced the time pressure on the Typhoon upgrade programmes. But there are not enough people and not enough money, so a pointless argument.