This is intriguing: Google results - UK pages only -
"I had better" 118,000
"I'd better" 225,000
"I better" 172,000
That is, the total of "I had" and "I'd" hits outweigh "I better" 2:1 - even assuming that all those "I better" hits are relevant. They're not. In the first screenful I found
not one that was relevant. There were three from either American- or Australian- related pages (presumably written either by speakers of something other then BE, or by people influenced by non-British factors - e.g. family connections); and one totally irrelevant coincidental collocation ("How can I better understand....").
In the case of the first person, I have no doubt that 'I better' is non-standard as far as BE is concerned. I'm not convinced the same is as incontrovertibly true in the case of "you better". Very few of the UK-page "you better" hits are relevant - on the first page, most are coincidental collocations. Most of the rest relate to modern pop culture - the first conduit of language change in a wired world. I don't have the time to look further into the second-person results, but it looks as though the increase in acceptability (which I don't dispute is happening - it'll just happen over my dead body
) is progressing via "You" forms to "I" forms.
I'd like to look into this further, using BNC rather than Google. But it'll take me a while to learn to drive that properly - time that I don't have at the moment (as I have an interview to prepare for).
b