You might want to check this out carefully before risking it. I am not sure if the boarding card QR code, including on the app, has restricted validity (i.e they are not intended to be used as multiple entry for security reasons) and I am sure you would need to ask a BA or HAL member of staff to escort you back through arrivals, including possibly passport control. I think going airside is intended for those who are actually flying rather than to facilitate access to work in a BA lounge.