Well here's one organisation's attempt at distinguishing between the two
http://www.builtincookers.co.uk/
But a single built-in oven and a built-under oven appear to be the same thing to me.
as i understand it, built in or built under still need a housing. and you can have the hob somewhere else in the worktop. so even if you have a built under oven, the hob doesn't have to be directly above it.
i'd agree with buildersmate, they're one and the same thing.