In reality, I would advocate a mixture of the three.
1) Raising retirement age makes sense as life expectancies have risen considerably over the last few decades, whilst retirement age has barely moved. This has obvious implications for funding the government as it increases the number of non-workers in relation to workers. It won't ever rise far enough for political reasons, but given the demographics of the nation, it's something I would do.
2) Fertility is a problem throughout most of the developed world, and whilst the UK isn't as bad in this regard as Japan, Italy et al, our birth rate is still below the replacement rate. More pro-fertility policies would seem to be sensible therefore.
3) I've advocated numerous times for open immigration policies. Society benefits enormously from diversity, whether from a cultural, intellectual or economic standpoint. As we're primarily talking economics here, the vast majority of migrants arrive as working age people and have a higher employment rate than the native population. They also statistically have fewer children and return to their homeland before they retire (the opposite of Brits retiring to Spain if you like). They're an economic boon.
Brexit will reduce #3 significantly, so that kinda leaves #1 or #2, but of course even if you implemented #2 today, it will take ~20 years before the children born today are economically active.