Anwulf: the "nother" from OE "nōhwæðer" isn't the same word as the metanalyzed variant of "another".

ppp: The process where "another" became "a nother" is called metanalysis, or recutting. The metanalyzed "nother" is in the OED, and it has been around along time. The OED has this citation from c1330:

Ich am comen her‥To speke wiþ charles‥& wiþ a kniȝt þat heet Roulond & a noþer hatte oliuer.

I have never seen any evidence that English or any language is worse today than it was a thousand years ago. If language has been progressively dumbed down, how can we still communicate? How do we define language deterioration exactly? What does an undeteriorated language look like?

Hairy Scot: It seems that if some changes are detrimental, speakers are compensating somehow. Language has been changing for a very very long time, and presumably we can still communicate as well as people hundreds or thousands of years ago.

It's in the OED: "Suitable for tailoring; able to be altered or adapted."
The earliest citation is 1917. And there is these citations:

1987 Lit. & Ling. Computing 2 232/1 The layout of the bibliography is determined by tailorable style sheets.

2002 B. Detlor & A. Finn in Å. Grönlund Electronic Govt. vi. 108 In terms of tailorability, citizens need to be able to personalize the information content displayed on a government portal so that the information matches more closely to user interests and needs.

If it's good enough for Luke Skywalker...

Standard English: what it isn't
http://www.phon.ucl.ac.uk/home/dick/SEtrudgill.htm

Students don't pick up their teacher's accent. It's possible that they might acquire their teacher's grammar, but it's not obvious to me.

I could say more but I'd only be repeating the stuff I've already written about register, context, etc. I like Derek H's point that sometimes teachers have more important things to teach than how to use grammar in formal writing.

Complaints about "improper" English seem to ignore register. What is correct in an informal conversation might not be correct in a formal written essay, and vice versa. AnWulf and Brus seem to be saying that there is one correct Engilsh to be used in all contexts, at all times. Surely it would be more helpful to students to teach them about register, and how to use the language appropriate for the register.

How much language do students acquire from teachers anyway? I'd say not much. Students acquire language at a very young age from their peers. They don't grow up speaking the same language as their parents or teachers. This can be easily shown: children of non-native English speaking parents speak with a native accent, they don't speak with the same accent as their parents. And children who are taught by a non-native English speaking teacher do not automatically acquire a non-native accent.

Hold on... I didn't say "encourage". I simply hold the apparently weird belief that there's nothing wrong with teachers talking to their students in the local idiom. Correctness is not absolute after all. It depends on region and register. Such a teacher could still say "when you're writing formal English, don't use 'was sat'".

If it is a common part of British English, then I don't see why British English teachers shouldn't use it in a British English classroom.