3. - Not a problem. There are enough sprogs being born right now to deal with the short term. In the longer term, robotics is advancing at such a pace that decent human simulacra will be available thereafter. In fact, I wouldn't bet against such androids being available before the babies born now reach working age - and that's the real problem. Oversupply of humans for a dwindling number of jobs, which will result in societal unrest caused by bored, unemployed, humans. As to the pension funds, the cost of living will likely continue to drop as 'cheap' robots replace 'expensive' humans in many roles.
4. - Immigration isn't the problem and never has been, despite the whinging of the brexiteers. The vast majority of the UK workforce was born here (surprise surprise), but (alas) many of us Britishers seem to have a peculiar sense of entitlement and want to land in a well paid plum job from the off, rather than working their way up to it. And if the plum ain't available from the start then they'd rather claim benefits. So it isn't really surprising that less entitled, more willing, bodies from other parts of the globe should be employed in, let's say, more menial roles whilst the lazy Britishers lie on the sofa, watching Jeremy Kyle, and screwing the pooch with their Brexit votes. That said, I do feel that anyone immigrating to the UK should be subject to the same rules on child count as everyone else - so don't come to my dystopia (a dystopia being the practical implication of a utopian dream - it never quite works out!) with more than two kids unless you can afford to pay child tax, or have no objections to being sterilised.
6. We never lost it.