]Firstly, that's quite a turnaround to go from "everyone has the right to follow whichever religion they choose" to "you must follow Islam". This is a democracy. They'd have to have a huge proportion of the population that not only follow Islam but believe this is the right thing for everyone else to believe also (most Muslims don't: they, like Christians, believe that their faith is correct, that's all). You'd have to have a huge proportion of people who don't believe in freedom of religion, leave the EU etc. It is VERY, VERY unlikely to happen and certainly won't happen in the next 100-200 years.
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Again, I'm really not seeing how a change in the proportion of the population following a certain religion leads to this. You're talking about a minority of a minority religion (globally). Religion doesn't tend to come into UK politics -certainly not to the extent that it comes into US politics - anyway, so that would have to change also. Besides, the Koran preaches peace and tolerance, and that above all is what people of all faiths and none want.
An ageing population is likely to mean that everyone pays slightly higher taxes, rather than a change of religion?
What's FAR more likely is for the UK to be declared a secular state, like the Netherlands, since the number of people of all religions in the UK, particularly Christianity but also the many, many other faiths practised by people in the UK, are declining.