I suspect that the answer is that, for historical reasons, there are a large number of people of South Asian origin in the UK and many fewer of any other sort of Asian origin.
4.9% of the population in the 2011 census described their ethnicity as "Asian or Asian British" and chose the subcategory Indian, Pakistani or Bangladeshi, and a further 1.4% chose "Other Asian" (there is a separate category for Chinese, 0.7%). Many South Asians identify as Other Asian and not with any particular South Asian country (particularly common among Muslims who, or whose ancestors, immigrated from what is now India), so it's fair to conclude that ~5% of the UK population is South Asian and only 1-2% are any other sort of Asian.
The US race question in the 2010 census is different from the UK ethnic-origin question, but the overall Asian-American population is 4.8%. I could not find a breakdown, but Chinese and Filipino-origin Asian-Americans are the most established and largest communities.
"Asian" has come to be used to refer to the prototypical Asian in both countries - which is a South-Asian origin in Britain and an East Asian origin in the US.
British people, incidentally, are much less likely to use "Oriental" to refer to East Asians than we were in the 1980s, because we've absorbed the sense that it's offensive from American media. We might use it if fishing for a term - "East Asian" is not a category that comes to mind because it's not one that Brits deal with much, and the overwhelming majority in the UK are Chinese (ethnically; many Chinese in the UK immigrated from Chinese-minority communities in former British colonies in South-East Asia).
First, British English speakers do not all pronounce things the same way. There is a lot of dialectical diversity in Britain. As you saw in the comments, nickson104 is a Welshman and he pronounces "want" a different way from what I describe in my answer. No one pronunciation is absolutely "correct," since you'd need to specify in what context you mean it. However, I guess you mean to ask about the usual pronunciations for foreign learners in particular. That makes things easier, since resources for teaching British English as a second language tend to converge on a standardized or generic variety. Throughout this answer, I'll refer to "standard" AmE and BrE, by which I basically mean the forms that dictionary pronunciation guides and the like tend to record (and that foreign students tend to learn in classes).
How "want" is pronounced (according to a British dictionary)
The Oxford Learner's dictionaries list the British pronunciation of "want" as /wɒnt/.
Context on what these symbols mean
The word "can't" is not pronounced with /ɒ/ in any dialect of English, as far as I know. It seems you mean [ɑ]: this is the vowel sound generally heard in "father," "car" (if you pronounce it without an "r" sound), and the American-accent pronunciation of "not."
In standard American English, the vowel in words like "lot, not, stop, robot" (we name the vowel in this class of words "LOT" for short) is pronounced the same as the first vowel of the word "father" (named FATHER); the symbol for the American pronunciation of both LOT and FATHER is [ɑ]. For this reason, many pronunciation guides based on American English don't use the symbol /ɒ/ at all, and always use /ɑ/ instead.
However, in standard British English, the vowel used in FATHER words (like "father") is different from the vowel used in LOT words (like "not, lot, stop").
The FATHER sound, /ɑː/, is pronounced pretty much as in the American pronunciation of the word "father." The vowel is "long," literally: it is generally held for a longer period of time than the LOT sound (that is what the symbol /ː/ means after a vowel).
The LOT sound, /ɒ/, is pronounced somewhat like a short version of the vowel in American "father," [ɑ], but more like a short version of the vowel sound in American "law," /ɔ/; if "law" has the same vowel as "father" for you, use a short version of the vowel sound in "fort" instead, but remove the "r" from the end of it.
(These are just approximate descriptions; the important thing is to be aware that there is a difference so that you can listen for it and learn more about it if you need to.)
The word "can't" has the long [ɑː] sound of "father"; it does not have the short [ɒ] sound of "lot". The word "want" has the short [ɒ] sound of "lot"; it does not have the long [ɑː] sound of "father".
General advice for studying pronunciation
Google doesn't pronounce words very naturally and sometimes has mistakes. A better way to learn pronunciation is to study English phonology so that you know what sounds and distinctions to listen for (especially the distinctions that aren't in your native language), then listen to native speakers in Youtube videos and things like that, and if you want to know how to pronounce a specific word, look up how it is transcribed in a dictionary that gives pronunciation and listen to it on a site like Forvo.
Best Answer
At least Britain seems to have largely aligned itself that way. Quoting Wikipedia, which has an excellent entry on the topic (Long and short scales):
I wouldn't say that the world has done so, however, as the list of long-scale countries is rather long. For example, in Finnish the only word we use for 109 is miljardi, while 1012 is biljoona.
But you could well say that about the English-speaking world which now almost universally uses "billion" for 109, according to that Wikipedia article.