Upvote:0
How long is a piece of string?
This will depend on the fare class of the ticket and the booking site and even the same booking site might have different rules for the same flight on different dates due to the fare class they can source from the airline at that time.
As a general rule of thumb ( there are exceptions ) trying to βfool the systemβ is never a good idea and will eventually bite you in the behind.
Upvote:2
I have done that many times, with many airlines (although not with BA), and it was never an issue.
Generally, if you have a valid ticket, the airline gladly sells you additional luggage allowance, that cost you between 30 and 120 $ per leg, rarely more.
You can also simply show up with luggage, and pay at the check-in counter, but many airlines take a higher price for that.
Note though that I found never a third party booking that was cheaper than a direct booking with the airline. Typically, they only seem cheaper, until you try to checkout, and they add funny fees or taxes.