Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
If you spend more than 183 days in the UK in any one tax year you will be liable to UK taxation on your world wide income. In any event it is normal for persons in receipt of pensions to be taxed in the country of origin irrespective of where they reside.
Portuguese tax rates at your level of income run up as high as 48%, see here for a table:
Remember that these days the UK is regarded within the EU as a tax haven. Thus, although under the Double Taxation Convention between the UK and Portugal, the same income stream cannot be taxed in both jurisdictions, if you move to that country you will be liable to their income tax on top of the UKs. The Convention allows any tax deducted by the UK to be allowed as a tax credit against Portuguese Income Tax, but it does not protect you against differences in rates of taxation so if you move there you are still going to be paying more.
If you do move there be sure to complete a Form P85 and send it to HMRC. That Department will then classify you as non resident for the tax year after your date of departure and furthermore split your leaving year into two portions, one resident and the other non resident. Once so registered you may spend up to 91 days in the UK in any one tax year without breaching your status. However, being classified as non resident will not make a halfpenny's worth of difference as your pensions will still be taxed under UK rules and be caught under Portuguese rules also as I explained.
That is the position. I am so sorry to have to rain on your parade.