Dave... I believe those vents are there as the door is small and this was a Guest house above and this was the cellar for stock etc.... I have been in the cellar of the Sewing machine shop as that where my girlfriend in1964 lived, she didn't live in the cellar, and it was cool so an excellent storage space.
The toilets mentioned earlier were in the part of the Banks the Chatham side of the bridge, the vents were similar but smaller.