Disenfranchised in the 21st Century

In a modern Western democracy how can a huge number of honest, tax-paying individuals find themselves outside the voting process, disenfranchised and voiceless?
This is the situation facing thousands of British expats living in Spain right now who fall foul of the gap between UK and Spanish legislation.
Elizabeth is an English teacher from Kent who moved to Catalunya seven years ago. Since she came to Spain she has maintained her postal vote in the UK, but in eight years that will expire and she will no longer qualify to vote in British elections. While she is able to vote in local Spanish and European elections Spain only allows nationals to vote in general elections.
So where does that leave her? Quite simply, without a vote.
If she wishes to vote fully in Spain Elizabeth must become a Spanish citizen, if they accept her. It is a complicated, lengthy legal process which also involves formally renouncing her British nationality in front of a judge and swearing allegiance to the Spanish crown.
The problem is that, while Britain allows dual nationality, Spain does not, except with a few Latin American countries and former colonies like the Phillipines. And this question of dual or multiple nationalities is at the heart of the matter.
Let us look at the case of Martin, a British citizen living in Melbourne, who recently took up Australian nationality. He was able to do so without renouncing his British citizenship, because Australia and Britain both recognise dual nationality. Martin is now eligible to vote down under. (In fact he has to, as voting is compulsory there, but that’s another debate entirely.)
But what about Spanish expats?
Spanish citizens who move abroad have the right to vote in Spanish elections for the rest of their lives and even their children and grandchildren have claimed these rights through inheritance. So is it right that Britons who live in Spain, pay their taxes and social security there and generally help the Spanish economy should be penalised and voteless while descendents of Spanish expats and members of former Spanish colonies, who have never set foot on Spanish soil, can claim their nationality and right to vote?
It all comes down to nationality. Living in the 21st century where people regularly move from country to country shouldn’t it be possible to be a British national living in the country of their choice but with all the rights and penalties of that country? Why does nationality have to equal state? We are all people of the world. Why does where we were born matter so much?