I liked the home-grown helicopter at Cranfield a couple of years ago with the colourscheme that changed depending on where you viewed it - G-ESUS.
There was also the Berkut with reg G-REDX, and on its first appearance, the X was really red. But the humourless CAA put paid to that, and he was told to apply it in black. Nice guy, believe he was an engineer with Lotus (Glenn X?)
BA (and I believe BCAL before them) had a DC-10, G-DCIO, and there was a Tristar at STN G-IOII.
D-IANA was a Cessna 310, and/or Beech 200, D-ICKY a Duke, I-TALY an MB308, OO-OOO a Falcon 20. SE-XEE was a Stolp Starduster, and SE-XIZ an Extra 300.
Hot Air Balloons D-OLLY, D-ONAU, D-ONKY, D-ONNA, D-ONUT, D-ORIS, D-OTTY, D-OVER....
Wonder if there was ever an I-DIOT?