The correct charge for a WT seat is £20. If you were changed £30 as you say you were, you were overcharged. Note, exit row seats go for £50 and there is no specific tariff for window pairs. So, it would be worthwhile checking your receipts/card statement to verify the exact amount you were charged.

Don't get me wrong, i believe your argument to be a valid one. BA presents its customers with a premium proposition which is intended to provide peace of mind, not disappointment. I'm a cynical git hence the T&Cs were my first port of call after reading your post. As the T&Cs are written, emphasis is given on the individual customer rather than any family or group, which is why I considered the suitability of each seat individually. This is a very narrow interpretation, I grant you.