I am sorry, I miswrote!
- for those not on a PT contract and those who are not living in the UK, tax is paid in Portugal at a certain percentage
- in most EU countries employees only pay national social security fees and a small amount of additional national tax on top of the PT-tax
- in some EU countries, employees only pay national social security fees and no further tax
Regarding the question by joeporto: you definitely pay your social security in the country where your gateway is defined. Additional tax depends on the country you are based in. Basel and Geneva are special cases, as they are bi-national airports. Bottom line: you cannot be based in France, so you'll need to have an address in Switzerland to use Basel or Geneva. Therefore you also pay your social security in Switzerland.