"That's ace" would be the same as "That's great".
"I bought you a present", "That's ace!" - meaning "it's good"
"Shall we go for a meal?", "yes, that's ace" - meaning 'yes i agree, let's do that'
Ace would work in both examples, though it has a slightly more exciited feel to it, as if it's even better than great.
There may be other examples, "I aced my exams" maybe would mean 'i passed easily'.
I dont know where it comes from. Maybe tennis or the ace in a deck of cards or something else maybe.