I've been doing a little research and there may be only one option. Here were my top contenders for oil drain valves.
Left: EZ Oil Drain Valve. Center: Fumoto Drain Valve Right: Femco valve.
The EZ and Fumoto valves operate in the same way, except for the finish. I like that the EZ Oil valve is plated, and has a nice rubber cushion on the valve handle. The handles on both valves are spring loaded into a detent, requiring that they be lifted before they can turn. I think the likelihood of them opening unintentionally is remote. There are other valves I've seen for stationary industrial machinery that don't seem as secure. The versions I've pictured above have an accessory elbow, which I assume anyone with a Defender would choose.
Then there's the Femco drain valve. This is what I believe Difflock.com carries. To use it, first the cap must be removed. The oil doesn't begin to flow when the cap is removed. The oil doesn't begin to flow until you screw on the accessory drain hose. (This means, of course, that you have to buy the accessory hose.) The cap and the check valve provide redundant safeties.
A possible negative for the EZ and Fumoto valves is that you are at the mercy of the way the threads are cut into the oil pan and the valve. When the valves are screwed in, there's no guarantee that the orientation of the valve and handle will be optimal. With the Femco valve orientation's not an issue.
But all that is probably moot. Red90 above suggested that the thread size for the oil plug is 1/2-14BSP. I have no way to be positively sure of that other than to lay my hands on a bolt with that known thread size and try it, but I've looked elsewhere for confirmation and there is anecdotal consensus. I emailed the folks at EZ and Fumoto. EZ drain doesn't make a version with the right threads. Fumoto hasn't gotten back to me. But Difflock has version of the Femco with 1/2x14BSP threads, and they say it will fit a Defender in their product description
I'm going to order the Femco valve and a right-angle drain hose. I'll report back and confirm whether it's the right size.
The instructions for changing the oil are to remove the cap, screw on the hose, allow the oil to drain into a suitable container, , remove the drain hose, replace the cap, and refill with fresh oil. I might alter the sequence. It might be a good idea to put some replacement oil in before replacing the cap, and check for any oil seeping out of the check valve - to be sure the check valve is not being held open by any debris.