A mobile phone charger typically draws about 0.25 watts of power when plugged in but not charging a phone. So it would need to be left plugged in for 4000 hours in order to use 1 kWh of energy. As the price of that amount of energy is currently capped at 28p, that works out at 0.007p per hour, which is equivalent to about 60p per year.
The energy consumption when actually charging a phone is a bit higher but not a lot. So leaving a phone charger permanently plugged in, while occasionally using it to actually charge a phone, probably costs about £1 per year.
It's still a good idea to unplug a charger that's not in use because of the slight risk of fire if anything shorts inside them. Similarly, the advice from fire brigades is that appliances should never be charged overnight - but I'd be prepared to bet that's exactly when most people do charge their phones!