For the trip to Canada, I would be Canadian in all respects. So I would fill in your API as a Canadian and have any VeriFLY / Salesforce data lined up with that. Husband hasn't got a choice, but that's fine, it's not unusual. For the return to the UK it doesn't make much difference, you could stay Canadian (to save changing your API data) but then use your UK passport at the e-gates, the UK isn't so bothered about lining this up. You can of course use your Canadian passport at the UK e-gates too, but in theory this could lead to a discussion about the purpose of your visit (etc). It is just possible that an airline staff member in Canada would want to see your UK passport, purely because in one sense it looks like you have a one-way ticket to the UK, but the UK passport would establish your right of residence. Unlikely to happen.