Matt,
The idle speed change after warm-up is pretty normal. Oil viscosity changes, running clearances change, and even the mixture will respond to different temperatures of operation. Generally, we warm a car up thoroughly and then tune it. An early B, depending on its condition, is one we try to get to idle at just under a thousand RPM or so, consistent with smoothness.
What worries me more in the case of your car is that when you depress the clutch the engine almost stalls. Now, there are many possibilities here, but if the mixture and timing are correct, and the carburetters are in good repair, this should not happen. I'm wondering how long you've had this car and what you know about its mechanical history. When the idle drops dramatically upon declutching, as in your case, it can indicate that there is an awful lot of mechanical drag being introduced. Possible causes are a bad release bearing worn down into the cast iron holder, or a bad input shaft bearing or spigot bearing. Is there any noise or sound difference accompaning this drop in RPM? Let's not panic. It may be nothing of consequence. But, let's look into it.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon