Part of it is probably that there is no protected constitution in the UK - there is no distinct set of laws set up as "the constitution", and therefore there aren't any laws which see special protection from changes. Moreover, privacy in general is a relatively new idea, and the lack of political upheaval over the centuries means there haven't been any major events to cause a general overhaul where such privacy laws might have been created.

Part of it is probably that the scaremongering from mid-market and tabloid print media, as well as the ruling party actually works, and people have been led to believe that the authoritarian measures are for their benefit. There seem to be fairly widespread xenophobic attitudes, and a fear of the "out of control" youth.

The education that kids receive at school seems to be pretty poor [1], which gives people little historical and political perspective and therefore no strong feelings towards personal freedom.

The plurality election system of the House of Commons also causes a relatively weak opposition. I haven't been following UK politics recently, but as far as I know, the second most powerful party, the Conservatives, don't oppose the authoritarian measures, anyway.

[1] I studied at a UK university - the average standard of education among my fellow students seemed far lower than what I received in Austria