how many

The population of the united kingdom at the start of the outbreak was roughly 60,441,457.

The problem being that each new zombie on average would infect and create 3 more, causing an exponential increase in their numbers. Predicted infection figures from the experts early in the out break, before all news and media stopped, were that at least 99.999% of the population would be infected before the infection rate would begin to slow, but would only stop once the entire population was either infected or dead. The infection rate was predicted to reach that percentage within 2.5 weeks.

Whether any of this is true is debatable. I am not a statistician.

Based on these figures could mean that there are (bearing in mind some of those infected have been destroyed and many took there own lives before they could be) 60,440,200 zombies outside. I would say that this is a conservative number.

For a idea on how infection spreads have a look at this zombie infection simulator, it obviously is not entirely acurate but does convey a good idea of how infection spreads.

With that in mind, that means there are approximately 500 survivors also out there somewhere.

I hope you stay one of them.