From the Inland Revenue:
http://www.hmrc.gov.uk/pdfs/ir20.htmEspecially section "Earned income arising outside the UK", but you may want to read other parts of the document.
It is a bit long, but it is the IR, so it should be correct.
If you are resident, ordinary resident, and domiciled in the UK you should declare all your income. In those circumstances, it would not make a difference where the income was earned or whether or not you brought it back to UK. However, it would make a difference in other circumstances.