As a Yank and a complete newbie to coin collecting, I feel exceptionally qualified to tell you what your top five iconic coins are.
The first is a given: the Two pence 1797 Cartwheel. It's interesting from its historical importance as the first mass-produced coin; its monstrously huge; it features an iconic monarch, George III; and it was minted on equipment designed by James Watt, for God's sake!
The remaining four should feature iconic denominations (farthing, penny and shilling) and/or the remaining iconic monarchs: Henry VII;Liz I, Victoria, and Liz II (Sorry we haven't really heard of any of the others, though I know you had quite a few Georges and Edwards...).
So how about a Queen Anne Shilling (yes, no one has ever heard of Queen Anne, but these were made when Isaac friggin' Newton was Master of the mint--the most important, iconic figure to ever be involved in coin production...). I think the first GB shilling is more iconic, but I'd accept the last English Shilling if you prefer...
Lets take an 1838 Victoria young head farthing next, crossing off both an iconic denomination and monarch...
An Elizabeth I groat is my fourth choice. It's Liz I and its a groat! Everything went downhill for you guys when you stopped making groats...A Henry VIII groat could be substituted, I suppose...
And finally, and I know this will cause any serious coin collectors to sigh, how about a 1967 Elizabeth II penny? Last of an era, and the large penny is your most iconic
denomination of all...