As I understand it, you can drive on a regular Spanish license for up to 3 years after becoming resident in the UK, or until the age of 70 (whichever comes earlier). You can exchange your Spanish license for a British one at any time, and you don't have to take any form of driving test to do so. See
http://www.dvla.gov.uk/drivers/drvingb.htm for further details.
When a visitor, you can bring a fully taxed (elsewhere) vehicle from elsewhere without payment of any UK road fund or other taxes. Paperwork and approvals for a permanent import are a little more complex - best to look at
http://www.dvla.gov.uk/vehicles/exptimpt.htm rather than have me paraphrase it. If the vehicle isn't new (or close to it) I don't think you'll get stung for tax. Our experience of the offices / people involved with this stuff (importing a vehicle from the USA) was unpleasant - of all the aspects of marrying / moving from the USA / getting Visas, etc, this was the one that was the most trying.
A further comment - a left hand drive vehicle may be inconvenient at times. A little tricky to overtake, people look at the passenger for visual signs at roundabouts and other give-ways, and getting out of car parks the ticket machine and barrier are always on the wrong side. We we quite happy with Lisa's LH drive for years but do give it some thought. Much more about this in other threads here
