Here are some pictures of the MGA Coupe door latch and lock mechanisms.

Right side door (handle and link incorrect position, see below).

Right side latch (no key lock).

Left side latch (with key lock).

Above: Left side door latch, pull strap and inside handle pivot, inboard and outboard views.
Above right: Inside handle pivot mount inboard and outboard views.
Right: Left side door latch inboard and outboard views.

Above left: Left side latch unlocked. Above right: Unlatching with outside handle.

Outside key lock locked and unlocked.

Inside handle level at rest, raised to lock, and push down to open.

SPECIAL NOTICE: You may realize that inside and outside locks are two different and entirely independent functions. It is possible to raise the inside handle to set the inside lock, and then close the door, and it will stay locked. If you do this with both doors there will be no way to get back into the car, not even if you have the key in your hand.

On 7/8/2018, Jean-marie Auzon wrote:
"It happened to me one month ago. After a panic moment, I succeeded in slightly opening the quarter light glass by inserting a screw driver, sufficiently to insert another screw driver to pushing the 1/4 light locking finger. Then it was possible pass the arm and unlock the door. I had the chance to have several MGs around with all types and sizes of screw drivers".

To avoid locking yourself out of the car, it may be prudent to disable the inside locking function on the driver side door (or the door with the key lock on the single lock cars). I can think of a few ways to do it. One easy permanent modification would be to cut the nose off of the "rocking duck" that stops against the bell crank arm (when the inside handle is raised). Another way would be to install a screw in the latch body just above the bell crank arm to lock out upward motion. It may also be possible to drill and cross pin the inside handle pivot assembly to allow motion only in the "unlatch" direction but not allow the handle to move upward.

After these suggestions Peter Tilbury looked at his parts on hand and came back with this photo and note. He wrote:
"When I checked my driver’s side inside door handle mechanism, I found it had a hole drilled in it. This might be original, or may have been drilled by the PO. So I fitted a split pin to it (see photo). Now the handle goes up slightly but not enough for the handle to lock the door latch. The handle still goes down to open the door. Bingo – no accidentally locking the car with the inside door handle. It can only be locked/unlocked with the key from the outside".

Then A.F.Enticknap in North Yorkshire, UK, wrote:
"Attached is a picture of the LH door lock on my ex-Californian Coupe, showing the little grub screw that disables the interior locking mechanism. It's easy to remove. I haven't yet checked the RH mechanism to see if it's pre-drilled but it can't be a big problem if it's not".

So it seems like the holes are there in both sides, just a matter of installing a screw to block the motion to disable the inside lock. I recon the inside lock should be disabled on the driver side (assuming it has an external key lock there). I suppose the passenger side is a matter of personal philosophy.