I have it. It's an extra service with BT. Works out at about £3 per month. Definitely worth it. Any unrecognised number doesn't get answered - the answerphone takes over. Saves all the junk calls.
BT charge for caller ID but uf you sign up for telephone prefrance service (which is free) it comes with caller ID as an extra so you stop those annoying sales calls and also get your caller id as a bonus if you phone bt they will set this up but be sure to ask for telephone prefrance with caller id because if you just ask for caller id they will change you unfortunitly i dont know if this is the same for other providers you would have to phone and ask hope this helps