"Surprise surges"? Why should passenger numbers be a surprise to UKBA?
They have access to passenger details, so they should know how many to expect at any given time. And dates of public holidays around the world are generally known years in advance, so there is no excuse for not planning ahead for days that are likely to be busy. (There will undoubtedly have been an above-average number of passengers last week-end because most of Europe had a public holiday yesterday and many companies were also closed on Monday)