We made the same mistake when booking GVA-LHR. Fortunately, before we finalized the transaction we noticed the no-baggage. We had to cancel out and go back to the beginning to select the slightly higher fare that included 1 piece at 23kg.
It wasn't that clear to us and I think improvements could be made (they may already have been - as reported by HT)
Still cheaper than either BA or those orange guys, for that day and time.