Cast your eyes upon something, figuratively or literally. Synonymous with take a look. Both forms are found in both British and U.S. English throughout the twentieth century but the charts below, from Google Ngram, showing frequency of use of have (blue line) and take (red) between 1940 and 2008, suggest some of the nuances. In Britain, below, have started the period way in front, then steadily declined until it was overtaken by take in the mid-90s.
In the U.S., the two were roughly equal until about 1960, when take took off and have went into a tailspin, only to revive recently as a NOOB:
Are these clothes worth it? Go have a look. If ever there was a season to explore the mind of high fashion, this is it. (William Norwich, New York Times, January 18, 2004)/Mitt Romney has turned black, at least according to a graphic that appeared on a Fox News program today. Have a look — this is strange even by the standards of this Republican race. (Erik Wemple, Washingtonpost.com, December 14, 2011)
notoneoffbritishisms.com 
OT: What are phrases like this that have the same meaning called? A prime example of opposites that have the same meaning would be “fat chance” vs. “slim chance”.
@Hal: I think opposites like that is called antonymy. A word with opposite meanings is an antonym. It could also be classed as irony.
Meanwhile: in British English, take or have a butchers means to take or have a look.
Anywhere yet in US English?
Derived from Cockney rhyming slang – butcher’s hook = look.
I get most of my Britishisms from TV, mainly PBS and BBC America. “Take or have a butchers” is totally new to me. Never heard it before.
Meanwhile, two of my pet language peeves are 1) the nearly universal placement of “only” too early in a sentence, so that it modifies the wrong subject (but it’s become so common that everybody gets the meaning), and 2) the interchange of subjective and objective prononuns, particularly of “I” for “me”. I suspect these usages originated in America, but I’m now hearing them commonly on British TV programs.
I would not blame Americans for for bad English – the British have become masters of it themselves.
Unfortunately educational modernisers in the UK have held sway these past four decades and decreed that grammar, spelling, punctuation were no longer important. Children should not be taught to read and write but allowed to learn in heir own way in their own time.
And they have which is why illiteracy levels are so high.
TV dramas supposedly reflect the language used in real life, but the truth is the writers do not themselves know the difference between what is correct and what is not.
We hear TV presenters saying things like, “I was sat on the bus” or “I was stood in the airport” to which I ask, “By whom?”
My pet hate is the tyranny of “pre-“. Pre-planning as opposed I suppose to post-planning. Cakes must go into ovens pre-heated to a certain temperature because putting them in an oven heated to that temperature simply will not do.
My most recent favourite is a TV chef’s exhortation to “pre-prepare the vegetables in advance”.
Maybe the intent is to ensure that the heating has been completed before the next step. (The pedant in me feels compelled to point out that cakes do not go in ovens, heated or preheated, just as toast does not go in a toaster.)
But what’s the difference between pre-ordering and regular ordering?
We were taught proper English, even at primary school, and I remember SPAG marks making up a significant proportion of exam scores at GCSE and A-Levels about 10-15 years ago. That’s spelling, punctuation and grammar – at least 5% of every single exam, including non-essay subjects. Although inner-city London has recently developed a dreadful “in-yer-face” accent – “talking street” – yu get me innit blud? Apparently, it’s put on in some cases to “sound cool”, but they just sound stupid, and I read that one lad stopped talking like that instantly when he went to uni, which shows that it was put on at least in his case. Here’s an example of that accent: http://www.youtube.com/watch?v=TDkiFJZojpg
Not that I approve, but I would imagine that pre-ordering would be when one calls in an order from home, perhaps for take-home/take-away, while ordering is what one does while sitting in the restaurant after reading the menu.
Pre-ordering also makes sense if a product hasn’t been released yet. You can’t order something that isn’t out; you pre-order it, and they send it to you for release date. For example, hardware and video games.
…and other entertainment media, as well (BDs, CDs, DVDs).
I think “I was sat” is regional. I grew up in SE England and never heard it till I met people from other places.
Just a technical query – how come “have a look” has two separate entries on this blog, but the second entry doesn’t refer back to the first entry? Did Ben forget he’d already written an entry on it?