Correct - there hasn't been any left luggage facility at IAD since 9/11. Also correct that you will have to claim all checked items at US Customs. The on-airport Marriott will NOT hold luggage for non-guests. The BA counter won't open for 3-4 hours after your arrival into IAD.

Udvar-Hazy has lockers that will hold a medium size piece (perhaps a 21"-22" rolling piece). You need a US quarter to remove the key from the locker - and you get the quarter back when you return the key to the locker. If you have a piece too large for a locker, you can borrow a wheelchair (no cost) and wheel it around with you.

There is a bus that runs from IAD to Udvar-Hazy every 45 minutes starting at 10:30am weekdays - and every 60 minutes starting at 11:15am on weekends. Bus costs 50 cents.

PM me if you post more questions here.