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'Plumbers London'/'Plumbers in London' or 'Plumber London'/'Plumber in London' are more likely than 'Plumbing London' in a free text search. This would suggest that the word that describes the individual rather than discipline would be better (Dermatologist vs Dermatology), assuming they were going to choose just between those two.
I'm Looking For a [Plumber] in [London]
That is more likely if you have 2 boxes as phrased above. This would prompt Cardiologist (or the more generic Doctor) rather than Cardiology.
I would look at the search interface in use and decide from there. I'd also suggest that you never underestimate the ability of the general public to use an unlimited amount of terms for the same thing. E.G.
Someone needing a Dermatologist may search for:
Having a vast amount of local search pattern data you would be amazed at the amount of different phrases used. This gets even worse when you add in the fact that someone may search for a company or person by name, Google had a figure that I can't remember exactly but they say that over 50% of monthly searches are unique (I think it's quite a bit higher than that)
It's good to ask a few friends how they would search for things, remember to use a few different phrasings so you skew the results less. It's amazing how many people do not know what the 'correct' category they should ask for (hence the amount of cross references in the Yellow Pages).
I hope that helps a little
<<'Plumbers London'/'Plumbers in London' or 'Plumber London'/'Plumber in London' are more likely than 'Plumbing London'>>
This statement touches on the other question I've been working on:
widget/place (plumber in London)
place/widget (London plumber)
I've been leaning toward widget/place.