My husband (US citizen) is coming over from November 4th until Jan 21st 2015.
Can we take a cruise holiday in Spain and then return to the UK for the remainder of his visit?
I am a dual US-UK citizen. Before getting my UK citizenship, I have travelled to Europe extensively and visited UK a number of times - always without a visa. Same goes for Schengen countries, including Spain.
If you need official citation though, here is the link to the UK government website that confirms that US citizens do not require a visa for holiday/family visit to the UK. Note that without a visa, the US citizen can stay in the UK for up to 6 months (actually, 180 days).
Similarly, here's the link to the Spain government website confirming that citizens of USA do not require a visa to enter Spain on holiday for up to 90 days.
US Citizens entering on a US passport don't generally require a visa for the Schengen area or the UK. In the UK, if they have been previously denied a visa, it is recommended to apply for one.
Also the UK is not part of the Schengen area and Spain is. So the US citizen can enter Spain visa-free for 3 months and the UK visa-free for 6 months.
My wife is a US citizen and she has entered both those countries without a visa before.