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I don't get the six months thing? The rule is simply to decide who you pay tax to, either the UK or Spain, and it is decided by where you spend more than six months working.
It's not just working. I know some people who are very careful to not spend more than 182 days in the UK or they will become tax resident here and have to pay UK tax on their sizeable investment income. They complain as it means they can't see as much of their grandchildren as they would like and they also have to keep detailed records of entry and exit dates.
I don't get the six months thing? The rule is simply to decide who you pay tax to, either the UK or Spain, and it is decided by where you spend more than six months working.
And honestly, if you follow what I said above about residency, you'd be fine paying tax in the UK for the first year and winging it, before settling down, or not.