If you are resident overseas, the taxation of your pension income will largely depend on what type of pension is being received and the tax rules in your country of residence.
If you are residing in Portugal, UK State Retirement and Occupational Pension income will be taxable at the Portuguese progressive scale rates of income tax (10.5% - 42% in 2010). A small allowance is generally available before tax is applied.
Pensions arising from UK government service will remain taxable in the UK and will not be subject to tax in Portugal.
In the case of Personal Pensions, depending on how they are made up, it is possible to receive a significant percentage of your income free of Portuguese tax.