The right way to protect most BB is by dodging shells, but there are three notable exceptions: Lion1 (2 gun setup), Lion2 (2 gun setup) and Queen Victoria. The reason is that UK armor is the only type of armor that can bounce BB45 shells. With 10" deck you can bounce most BB1234 HE shells and 11" will bounce most BB5 HE shells. You should rely on dodging when fighting another BB one-on-one. However, if you rush in you will become the target of many BB1234: you need that deck armor to survive.
In GB, your primary duty as a L2 is to rush, not to line fight. You should point your bow toward the enemy to minimize your profile. No other nation on your team can spearhead the rush without dying. So I would put always deck armor on, treating it as a big asset for your team.