How, you may wonder, are such decisions arrived at? Well, it's all down to the regular meetings of the Filth Committee, who must remain eternally vigilant as this transcript of a recent meeting will show.
Chairman: Gentlemen, I think we can congratulate ourselves on having avoided the Viagra trap. One up for the DVLC there, I think. That might have been as disastrous for us as the Fellatio scandal of 1975.
Chairman: Ah, Marshall, you weren't with us in 1975, then?
Marshall: No, sir.
Chairman: Well, some bastard sneaked a number past us which read - well, I won't go into details.
Marshall: No, sir, I really meant - what is fellatio? A sharp intake of breath all round the table.
Chairman: Either that was a joke, Marshall, or it was a clear admission that you are incapable of performing this job. A dirty mind is essential in this job, Marshall - that and an overriding urge to interfere in other people's lives under the impression that you are improving them. That is why we get on so well with New Labour. Nervous laughter all round the table. Now, what's on the agenda today?
Secretary: Item No 1 is RSUXS, sir.
Chairman: Not quite with you.
Phelps: Perhaps I can explain.
Chairman: Ah, Phelps. You can usually be depended on to make an explanation grimly erotic. Fire away!
Phelps: Thank you, sir. Well, when an American doesn't like something he will say that it sucks.
Chairman: Sucks what?
Phelps: Just sucks, sir. It is always used intransitively. "Soccer sucks". Or "polenta sucks". But we get the unnerving feeling that some sort of obscene sucking may be involved, which of course would make any number plate incorporating SUX be much sought after.
Chairman: Right. And what action are you taking?
Phelps: We're on to the Americans, sir, for a bit of guidance, and we've applied to the International Obscenity Secretariat.
Marshall: The what?
Chairman: Oh God, I keep forgetting you're new here, Marshall. Fact is, we got quite good at keeping British filth off the road, before we realised we were letting a lot of foreign filth on. We saw to it in the early days that we had no car number plates with BUM or TIT on, and all the while we were letting cars go round with French words like CUL and CON on. Soon put a stop to that, with help from the IOS. OK, Phelps, get back to me when you find out about things sucking.
Phelps: Yes, sir.
Chairman: Meanwhile, put SUX on hold. Anything else?
Secretary: Yes, Item 2 is FEX.
Chairman: FEX? Not with you. Whose is this one?
Parry: Mine, sir.
Chairman: Let's have it, Parry.
Parry: Well, sir, you know that people are always trying to think of synonyms for the F-word, as American novelists went through a phase of saying "frig" and "frigging" even though nobody in real life ever said "frigging"?
Chairman: Yes, yes - get on with it!
Parry: The recent television smash hit Father Ted featured three Irish priests who tended to swear a bit. Well, instead of using the F-word, they were made to say "feck" and "fecking".
Chairman: Were they indeed! And what did the Pope have to say about that?
Parry: Nothing, sir. I believe the Pope has enough trouble trying to stop priests seducing little boys and having children, without worrying about them swearing as well.
Chairman: Fair enough.
Parry: Anyway, this means we are a bit worried about number plates incorporating FEX. It didn't use to be rude, but it may be now.
Chairman: Mmm. Interesting. Anything else?
Secretary: Usual request about SOD.
Chairman: The answer is no.
Secretary: Next item is NOB.
Phelps: Slang word, sir.
Chairman: I know that. A nob is a member of the upper classes. Nothing obscene in that, unless you're very left-wing...
Phelps: No, sir - nob, or knob, has recently acquired a new meaning... Etc, etc. Full transcript of this meeting on request.