My concern is sharia law. Currently the majority in this country don't advocate for sharia law but if the demographic changes significantly this may change. That would be perfectly democratic if the majority wanted it of course. But it's hardly something I would look forward to. I think people are concerned that many or most Muslims would support sharia law in the UK however and that perception might explain why some consider them a threat.
This is not a perception that I had growing up when I had many Muslim friends who behaved just the same way as anyone else re drinking, attitudes re sex etc so something has changed. The perception has changed in the last twenty years. It may be that the Islamist voice is louder now.