the use of the word "bare" as a British slang comes from Jamaican patois. I'm 100% sure of this.
I'm Nigerian, 36 years old, and moved to London at the age of 7. So I grew up in London. I'm familiar with virtually all the slangs that have come and gone, and those that still persist. In Jamaican patois, the word "bare" and "pure" can be used interchangeably to mean "plenty of". So "bare gyal" or "pure gyal" means plenty of girls. You can Youtube a song by the Jamaican reggae artist, Bounty Killer, titled "bare gyal ah mad ova me", which translates to "loads of girls are going crazy for me".
In 1996 when I was 16, white kids were definitely not using "bare" as a slang. It was a slang confined to the black community (that's not to say the odd white kid who had black friend's wouldn't use it.). Then Asian's (British Pakistani's, Bangladeshis,) and other ethnic minorities who were influenced or took on aspects of black British culture began to use it along with many other slangs from the black community.
I can't put a date on it, but I'd say it's only in the last 3 to 4 years that I've been hearing white kids using it. And I'm talking white people who, I can tell (from the way they speak) don't have many black friends.
Also, the meaning of the slang "bare" has changed a little in the UK. When I was growing up it only meant "plenty of". So "bare money", "bare girls". But these days, as someone noted above, it also means very. So "bare angry", "bare hungry". Before, people would have used the word "nuff" to mean very. So, "that chicken is nuff spicy". The word "nuff", again coming from Jamaican patois. Nuff = enough.
Another word from the black community (and Jamaica specifically) which I'm beginning to hear some white kids use is "chirps", meaning to "chat up". So, "go chirps that girl over there", or "that girl was chirpsing you, couldn't you tell?".