They pay tax, NI and the rest and have no say in who is at the top while being affected by the same decisions as the rest of the country.

Eighteen is the number of years it takes a British citizen to earn the right to vote; so why not residents? These people have also made Britain their home and should be given the choice of either voting in their country of origin (Often a society which they no longer have a stake in) or voting in the society to which they contribute.
This boils down to a case of no representation without taxation and no taxation without representation. These people pay tax and are not properly represented.