|With pins 11 and 12 jumpered, and with the round plug disconnected and in a plastic bag to avoid short circuits, the car will start and run normally. However, you will lose the functions of (a) the 'door open and key in ignition' buzzer, and (b) the 'fasten seat belts' warning light and buzzer.
If you're comfortable with altering the car's electrical system, you can remove all of the wiring associated with the infamous seat belt warning system and make a permanent connection between the wires that once ran to pins 11 and 12 of the round plug. This involves unwrapping and rewrapping the wiring harness, and therefore is most easily done when the dash has been removed for some reason. You'll want to save the blue tape used to wrap the wiring, as it can be re-used once cleaned of dust and grime. IIRC this tape is also available from British Wiring.
The 'door open and key in ignition' audible alarm can easily be restored. The buzzer used on European JHs for this purpose was still available from Delta Motorsports a few years back, and as it's unlikely this is a fast moving item, they should still have plenty on hand. Alternately, you could use a junkyard / aftermarket chime, buzzer, bell, horn, or whatever else suited your fancy.
The 'fasten seat belts' function can be restored if one adds the appropriate (but expensive) aftermarket gadgetry. Alternately, you could make contact with JH owner Frank Schwartz, who some years ago was looking into designing a circuit for the task.
However, a better use for the warning light might be to provide some new function such as indicating low oil pressure, low fuel level, hood or trunk lid ajar, door ajar, or whatever else suited your fancy. The legends giving the functions of the warning lights are thin plastic films, clear text on black background, and if you have access to a laser printer that will deal with transparency films, you can make new legends with whatever text you wish. Wiring, of course, would depend on what you had in mind.