A high alert threat level has just been posted in UK. what are they expecting?

"The new alert level means an attack on the UK is "highly likely", although Mrs May said there was no intelligence to suggest an attack was "imminent".

"It is the second highest of five possible UK threat levels.

"The home secretary already has the power, under the Royal Prerogative, to withhold a passport if it is in the public interest to stop somebody travelling.

"That power has been used 14 times between April 2013 and the end of March 2014 to stop people travelling abroad for alleged terrorist-related or criminal activity."

And here:
Terror attack on UK 'highly likely' as threat level raised - Telegraph