If you are resident in Portugal, you are liable to tax on gains made on worldwide property and shares (if acquired from January 1989 onwards).Only 50% of the gain is liable to tax for residents. Inflation relief is available after two years. The gains are added to your other income for the year and taxed at the income tax scale rates, up to 48% in 2018.

If the property was your main home and all proceeds are reinvested in another main home in Portugal or the EU/European Economic Area (where there is tax information exchange treaty in place) within 36 months after the sale or 24 months before, the gain is exempt. You have to live in the new property within six months of the end of the three-year limit. This could affect British expatriates returning to the UK after Brexit.

If you own residential property in the UK, you are also liable to tax there even though you are not UK resident. The net gain arising after April 6, 2015 is added to any other UK-source gains and taxed at 18% or 28% (depending on your UK tax bracket) with a £11,700 allowance in 2018/19). Under the UK/Portugal double tax treaty, a credit will be given for the UK tax paid, although you will pay whichever amount is the larger. Such gains may be exempt in the UK if you meet the conditions for private residence relief.

One of the benefits of the Internal Market is that EU citizens and businesses have the freedom to move, operate and invest across national borders. But since direct taxation is not harmonised across the EU, this freedom can mean that some taxpayers manage to avoid or evade tax in their country of residence which usually, according to the bilateral tax treaties, has the right to impose tax on worldwide income and assets, even if the income or asset is taxed in the other country. Naturally no double taxation should exist either, and there are agreements in place to avoid this, but the correct amount of taxes must be paid in each relevant country.

Tax authorities in the EU have therefore agreed to cooperate more closely so as to be able to apply their taxes correctly to their taxpayers and combat tax fraud and tax evasion.
Administrative cooperation in direct taxation between the Competent Authorities of the EU Member States helps to ensure that all taxpayers pay their fair share of the tax burden, irrespective of where they work, retire, hold a bank account and invest or do business. This is based upon Council Directive 2011/16/EU which establishes all the necessary procedures, and provides the structure for a secure platform for the cooperation.​

It appears to me that the OP asked whether CGT on a UK property would always be exclusively taxed in the UK

While capital gains from the alienation of real estate may under the double taxation treaty be taxed in the country in which the property is located and will therefore be exempt in Portugal...