When you move from one country to another, it is very important to plan ahead to ensure your income and assets will be structured as tax efficiently as possible. Leaving it until you arrive in your new country may be too late for some tax planning opportunities and you may pay more tax as a result. This applies whether you are moving from the UK (or elsewhere) to Portugal, as well as if you are moving back to the UK.
While right now you may have no plans to return to the UK, circumstances may change, sometimes unexpectedly. Many UK nationals do return at some point. Often the surviving spouse will return after the death of their partner. If and when you do decide to return, the most important tax tip I can give you is that you need to do careful planning in advance of your return.