I plan to live in Spain for more than 6 months. What implications will this have? Must I become a resident?

The current Spanish law states any individual intending to remain in Spain longer than 183 days in any one period throughout the calendar year, should apply for a ‘Residencia’. (A Spanish tax year is the same as a calendar year: 1st January to 31st December, unlike the UK tax year which is from 6th April to the following 5th April.). If you are resident in Spain you will be liable for income, capitals gains and succession duty (inheritance tax) on your worldwide assets.
It is also very important to remember that once you have obtained your ‘Residencia’, you should exchange your British Driving License for a Spanish one.