That's the delight of being British, there are written rules and then there are the unwritten rules of politeness and what my grandfather would have called "behaving decently".

In this case I think it would be perfectly acceptable for a husband and wife to book separate seats if they intend to sit in them come what may. But, if when someone takes the middle you immediately offer to swap, then you obviously do want to sit together, which is moving towards reserving a seat you don't really want. Which makes me uncomfortable.