Leno would be great however his schedule is pretty full since his shows are on almost if not always 5 days a week, every week of the year and they take almost all day to put together. To get to the UK may be abit of a stretch.
I can't remember ever seeing a stand in presenter on the tonight show.
Anyhow, he could get on a plane on friday night (saturday morning UK time though), do the SIRPC thing on sunday (the sunday before airing then I guess) fly home on sunday evening and be back at work on monday.... it's not impossible if he wanted to.
Svempa is right, Jay Leno is an absolute motorhead in every sense of the word. He's got a huge collection of unique classic cars and drives every one of them.