The columns could even be painted wood. They might also be some kind of composite material. It's impossible to say for sure without being able to examine closely.
John Nash, the architect behind many buildings in London like the ones surrounding Regent's Park, used stucco- Portland cement plaster.
https://bestgapp.co.uk/news/stucco-fronted-homes-stand-test-of-time/
I don't see any reason for the columns to be carved out of wood though it is possible. It will not last long under outdoor conditions and the cost would be prohibitive. The columns are more likely made of concrete which are moulded to the shape complete with embelishments. Stucco is more commonly used for plain walls in houses.
Also, for those wondering about stucco - stucco is a exterior application. It's a decorative covering, and can't be used to bear any weight. While you could stucco over the columns, the actual columns themselves would have to be made out of some other material.
Are there alleys in London where there are both stucco-fronted and brick houses?
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