I'm trying to work out why you collected your bags in LHR, normally they would go to EDI. Unless it was booked on 2 tickets. If you buy a ticket from IAH to BCN, you check the bag in IAH and then see it next in BCN. I'm guessing you have young children - the e-gates are now open to children 12 years old and above, but you shouldn't need to do this on a trip to BCN, you don't clear passports or customs in LHR on that sort of trip. Within reason you can go slightly over the cabin baggage dimensions, but if you do that you may find your bags checked at the gate anyway, or Flight Connections, and the way to avoid that is to stay within the advertised dimensions.
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