As others have said, getting to Spain on your UK passport should present no problems whatsoever.

The only problem I can see with your return journey is that you will have to deal with an airline AND the US authorities, and you will have to satisfy both of them that you are legally entitled to enter the USA.

Most countries let their own citizens back in on an expired passport, so your expired passport will probably be fine for getting past Immigration on your return - but you need to get on a flight first, and the check-in agent may well decide that an expired passport is not good enough.

You could get past the check-in agent by using your British passport for check-in, but to do that you would need to travel as a British citizen and apply for travel authorisation...

I am sure it can all be worked out, and talking to your consulate is probably exactly the right thing to do.

Best of luck.... and very best wishes to your father.