There was a Lotus Elise in Bridgwater, Somerset, back when I lived there, with
B10 JOB as the reg.
My brother (RIP) had my favourite plate ever on a rare little two seater race-prepped roadster (can't remember the make but again, was pretty rare and incredibly quick). -
BYE 1H
Extremely subtle, this one... it reads "HI" as it's coming up in your rear-view mirror, then "BYE" as it passes you (as it inevitably would).
He also had
B7 EKK (By 'eck) on a one-off RS Turbo Cosworth Cabriolet (built by Ford to order, originally - for some footballer, I think)..
Other notables I've seen in the past (apart from
A1, as I mentioned previously) -
1A UK (on a Triumph Dolomite, possibly a sprint),
BBC1 0K (on a BMW),
JES50P (on a Porsche at a Photographic Expo) and
DJ1 on some mediocre mid-range car (forget what but do have a pic somewhere).
Oh, I also saw
BA57ARD on a 911 when my brother was driving me from the airport to pick up my 190E.
Nicest plate I saw for myself might have been
PUG5 6 on a Rolls, many years ago in Solihull.
I have found an awesome reg still available at DVLA but I'll wait until I've saved up and bought it before I share that one. 
[edit] Nearly forgot
... Back when they first introduced them on the 'H' reg, there was a nappy delivery service that took
H20 POO (with the 2 staggered up slightly)
Oh and there's always this one (not seen personally but still funny, all the same).
