We are already seeing the effects of longevity and an ageing population on British society at present with assets (like home ownership or savings) and job security concentrated in older age cohorts. Furthermore, older age cohorts are disproportionately likely to require the NHS and other forms of social care for longer than younger cohorts at a time when the government appears set on privatising these services. There's the additional factor, that we do not know its effect yet, that the government changed the law last year so that pensioners could withdraw the entirety of their pension if they wanted to thus undercutting the cash reserves of pension funds that would ordinarily be used for incoming cohorts of pensioners. On wealth inequality, education and social attitudes it appears that the UK is strikingly divided by age (as well as regions) with this being a strong predictor for how citizens will vote in parliamentary elections or in the recent Brexit referendum. The UK welfare system was designed in an era when average life expectancy was 70-years old with retirement being at around 60 to 65 years old, and this model has not been significantly altered in spite of declining mortality (and fertility). Death from old age had the social function of redistributing property and assets to surviving offspring. It seems that this mechanism has broken down in developed states where the healthy demographic triangle has been inverted thanks to the longevity of "baby boomers" and their historic low fertility rates. This, of course, is a problem throughout the developed world including in rapidly developing countries like China and India.
The other factor will probably be immigration, which has been used to ameliorate the effects of low fertility on the productivity and supply of the workforce. If immigration will lead to a more pluralistic society or not has yet to be seen and will be greatly influenced to the extent that older (and, statistically, more xenophobic) generations are able to translate their considerable political capital into policies that either help foster pluralism or one that is hostile to it. It is doubtful that the government can bring down immigration to the numbers that they are advertising following Brexit without causing significant damage to the entire economy, which therefore makes their claims seem much more like pandering to certain parts of the electorate than realistic or sound policy-making.