The part that does the work
Two facts make this list possible
Every other step is engineering. These two are why the list exists at all, and why nobody sells it to you off the shelf.
One. In the UK it is a criminal offence to call yourself an architect unless you are on the ARB register. So the register is not one list of architects among several, it is the complete list.
Two. You need a registered architect to trade as an architect. So a practice with exactly one architect on the register is, by definition, a one-person practice. That is an exact size filter, and it costs nothing. It only works if you hold the whole register at once, which is why step 01 sweeps all 6,053 pages rather than sampling a city.