With everyone*, including myself, becoming bewildered with everything and harking back for the days when eggs were eggs, people were nice and chomps cost 10p, I remembered a few things I was told by different people as ways you could effectively cause Britain to grind to a halt.
Obviously some ways, such as nuking Milton Keynes or replacing the drought posters with signs saying 'WE'VE GOT FUCKLOADS OF WATER LADS, GO BANANAS!' may prove fruitless or too much and effort. However some may work.
My history teacher at college told me that if you orchestrated it so that a million people emptied their bank accounts at the same time then there'd be no money and the banks would be on their knees. I'm not sure this would work though, to be honest, and she had a Lenin bust on her desk and worshipped Eurovision. Still, a heroine she was. But I'm not sure her plan would work.
An old colleague of mine also said that if you were to get several lorries and park them on the main entry points on the m25 then the country would grind to halt. I could see that working for a little bit, but surely they'd get removed after a time.
Similarly, I guess you could also put obstacles across the main train routes around the country and bring the U.K. TO IT'S KNEES, or something. Would take a while to put loads across, but would cause a lot of grief I gather.
Anyway, what ideas do you have, bright young things?