Many people are unsure where to send a completed UK tax return. I always suggest sending a tax return electronically and not on paper for several reasons.
If you need to send in a paper tax return then you can send it to any HMRC tax office and the internal post will forward the return to your actual tax office.
Sending a tax return to HMRC means it needs to be processed manually and HMRC have huge post delays of several months and much post is lost. I always suggest you send post in by registered mail and ideally to a named person. This seems to help speed up processing.
If you have trouble locating a tax office address searching for 'find a tax office' on the HMRC website.