Cultures evolve.
Did British people do the same things in 1650 as they did in 1550? Probably not.
Is that because of immigration? No, it's because fashions and tastes change. New trends emerge. Societies are exposed to new experiences and they like some of them. Others don't catch on.
There's no dilution to worry about. If people wanted to have a street party on the Queen's Birthday or tune in to Trooping of the colour or eat toad-in-the-hole, they would.
If in 50 years, people aren't doing that, it won't be the fault of Bollywood or Moroccan restaurants or rap music. It's because the merits of these precious parts of our heritage didn't resonate with people any more. In the same way that, people stopped drawing on the inside of caves when they invented paper.
If people decide they'd prefer a salsa class followed by a kebab than bell-ringing followed by a Cornish pasty, more power to them.
Traditions, religions and movements don't die out because people are denied the opportunity to practice them any more. They die out because they cease to be popular.