I think they're pointless in the UK. Far too limited for the cost and you can't even 'personalise' them as such, you just get to pick from a list and half the time because of the letter/number compromise they look crap.
System should be like the States where it only costs about $100 to personalise it for the year and you can have pretty much anything you like as long as no one has already registered it and it's not blacklisted as rude/offensive. Best part is that even if someone else in another state has it you can usually still register it as long as no one in your state has it.
In the end you get a lot more personalised ones on the road so you see a lot more funny/interesting plates and it doesn't cost a fortune.