Sorry Johnny, Worked directly with Baxi with this one in conjunction with CORGI at the time, when I was a regulator in the industry. A CORGI Technical Bulletin was published at this time with regards to using the gas fire on a redundant back boiler. Baxi may have changed their view on this now, but the guidance was given in the 1990's and agree it is not an ideal situation but does not make it unsafe if you cap off properly and it passes all the statutory checks. Common sense used, because you don't have to have the BBU on if you use the fire under normal conditions. The reason for capping the boiler gas valve, is to prevent any risk of the BBU firing up when its not connected to the water side of the heating.
best to do what baxi says though imo,
it would be difficult to explain at an inquest, why you ignored manufacturers instructions, lawyers are not interested in common sense :yesnod: