The only way you can avoid metriculation tax requires complying with a tight set of rules.

Must have owned the UK car for more than 6 months (not too difficult)
Proof that you have left UK  
Now the killer
Produce a letter of local residency (Padron) I believe.  The padron must be your first one issued  and less than 60 days old.  Then you can apply for a get out of paying tax card from the Consulate

So anyone has had a padron issued say for getting a mobile phone or something similar seems a bit unfair.

The metriculation tax is on a sliding scale based on

A The Valuation of Said Vehicle
B A % of this valuation is then payable dependant on the vehicles offical CO2 emissions.

Very Low emissions  0 % payable then next level about 5 % up to the max of around 16% for gas guzzlers 4x4 ect

% Figures are approximate. My C4 modern family diesel hatchback with very good MPG just misses the free tax band of  below 120 CO2 thingys at 128 and I will have to pay the lowest of the 4 scales @ 4.75% of the valuation of my car for matriculaion tax amongst the other fees for road tax ect