The assistant was clear that she had to escort us to the loos and wait with us. I doubt she was breaking any rules, it wasn't a case of 'i shouldn't really, but I will just this once.' And had she said no to us, we would have understood perfectly.
Perhaps that particular B&Q was set up to have the loos just off the shop floor so taking a customer is not as much of an issue as the OP's situation?
In my current store to get to the staff loos you have to cross the warehouse, along the security corridor, up two flights of stairs, through the break room, through the separate mens/womens locker room and to the loos.
In the branch I worked in before this one (same company, different location) if you walked through the "employee only" doors you would be in a tiny corridor (entirely covered by cameras) with the clocking on machine; a coded door (which lead up stairs to the canteen/offices/locker rooms etc); two separate loos - one male, one female; and the door to the outer warehouse down at the end.
In my current store there's no way in hell I would risk taking a customer to the staff loo under any circumstance because it's not worth the risk, both from the customer getting injured and the security issues. In my old store I wouldn't volunteer to take someone to the staff loos but if there was an emergency situation then to get to the staff loo they would have to take 5/6 steps into the employee area (with zero risk of coming into contact with dangerous machinery, boxes, stock, equipment etc) which is much less of a risk (both from a security standpoint and a H&S standpoint.)