Most of the time it's down to the Atlantic jet stream.
The video below explains it well.
Notice how we get at the shit weather thrown at us via the jet stream, which basically sits under or on top of the UK, whilst Spain, which is underneath the jet stream get's the good weather. On occasions the jet stream moves higher up allowing warmer and better weather to move up to the UK, whilst the shit weather gets pushed up towards Iceland and above.
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Currently.
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