I'm a bit confused by your question!
You write : "why is it that the inland revenue state that they do not disclose your tax code to your employer" - My problem is that I can't think where you may have read/heard this! It's clearly not true for the very reason you suggested. (The purpose of a tax code is to tell your employer how much tax to deduct from your pay. Without the code your payroll section can't work out what to pay you!).
I can only think that you've become confused by the rules relating to second jobs. If you've got a second job and most of the tax is deducted from your first employment then HM Revenue & Customs (the new name for the body which replaced Inland Revenue) will only supply your second employer with a letter code which tells them how much tax to deduct. i.e. your second employer will not know your main tax code.
Chris