The + symbol represents an instruction to dial the international access code used in the country that you're calling from. In the UK (and many other European countries, but by no means all of them) that's 00.
By putting +, rather than any specific digits, telephone numbers can be expressed so that they'll work from ANY country as long as you know the relevant international access code. Most mobile phones now have a + button, which dials the relevant code depending upon where the phone is. (e.g. pressing + in the UK dials 00, while pressing + in Spain dials their international access code, which is 07).
Chris