I second this as a good place to begin the investigation.

Install a new oil pressure switch and see if it cures the problem if its not that then you are in to expensive territory.

Given how vital the oil feed is to the health of your engine, why not buy a cheap oil pressure guage off eBay and check that your pump is actually delivering what it's supposed to. Or take the car to a local garage and ask them to perform such a check and test the pressure switch too? Even if this costs you £100, it's peanuts compared to the possibility of there being an actual oil supply problem in your engine. Bearing or ring failure due to oil starvation will leave you needing a rebuild.

In the first instance, I believe in assuming that the oil warning light is doing what it is supposed to, not that it is faulty.

Most pressure switches will operate at around 5-7 psi. Obviously the running oil pressure will vary from engine to engine, but in the absence of model specific information I'd be looking to see at least 20psi at 2000rpm at normal operating temperature.

What's the condition of the oil like? Level ok? Is it changed regularly? Maybe you've got a partially blocked pickup?

Carry out your pressure check first. If this seems ok, perform an oil change with a full flush (a flush compound in the old oil, then run with a proper flushing oil in it for 1/2 hour, something like Comma Flush Out, then refill with quality oil. I also use a dedicated filter for my flushes). Then keep the pressure guage connected and keep an eye on it whilst driving, apart from dropping back at idle you should see the pressure remain steady).

I'm puzzled by the apparent lack of any investigative work to show that removing the sump is the next logical step.

Have you checked the working oil pressure? the operation of the low pressure switch? the wiring?