Personally I don’t think there is any justification for an enduring defence expenditure above 2%. Even now, the threat just isn’t there. However, because we are behind the 8-ball in so many areas, there may be justification for a temporary injenction of funds above that amount, until the appropriate capability baseline is reached. I don’t there will he much resistance from the electorate - we are already seeing a pretty massive injection of funds to get back to 2%, and the electorate has so far given that a stiff ignoring.
A significant problem to be overcome, however, is that there is just so much extra money that can actually be absorbed quickly. I can only speak from an army POV, but one key reason new capabilities are being introduced slowly is that there are just not enough people to run the programs simultaneously to bring them in any quicker. Injection funds quicker than it can be absorbed just risks it being squandered, with the predictable backlash being the result.
As to what we should be spending our money on if there is extra cash, I think we need to aim for quick wins. Most of the large programs won’t see significant capability being delivered for a long time, so looking at what can be achieved in the interim would be an obvious goal. I would aim to increase our operational-ranged strike options, and other related capabilities, as they ar the capabilities that will make a potential enemy sit up and take notice. A long range cruise missile to be integrated on submarines, ships and aircraft (including P8) would be a good start, as would a new anti ship missile, preferably LRASM, even if it was only a small number. SM6 for the surface ships, deployable long range rockets/missile/GBAD systems for the army, introduce an armed MALE UAS (which is well overdue) etc etc.