I was born in the UK, I a dual citizen UK/Australian returning to the UK after 20 years. I have a NI number, and will be drawing government OAP next year. Am I deemed a resident of the UK from day one of arrival?
It does depend, as phoog said, on what aspect of residency.
Generally speaking, you need to be in the UK for 6 months before being considered a resident. For example, you are NOT entitled to free NHS care for the first 6 months, so avoid mentioning your recent arrival to your doctor.
Tax is complicated, very complicated, More information is at http://tools.hmrc.gov.uk/rift/screen/SRT+-+Combined/en-GB/summary?user=guest
For opening bank accounts etc, you'll need a recent proof of address, but they won't check that you've been there for more than 6 months.