There is no walk up get a visa at the thai consulates in the west anymore (at least not at the ones that use the eVisa online system)
You can apply for a 60 day tourist visa on line thru the thai consulate in London using the eVisa system at this link;
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OR
you can come in visa exempt (the 30 day free entry stamp) and then take the one 30 day extension you're allowed on that at the immigration office where you stay for 1900baht