Boots occupies the building at the end of Mercery Lane. Actually, they seem to use such old buildings in many places in the UK. The frontage here is actually fake, having been redone early in this century. I believe however that they have some roman remains in a cellar which you can see if you ask. I've never done that though.
If you go on, you will end up in the main street of Canterbury about to enter St. Margaret's Street.