This 4-way flasher switch is from Classic MG Accessories who make them specifically for your stated polarity.

Once installed, all you see is a 15 mm diameter red illuminated rocker switch (wherever you want to put it). When operated this switch flashes and connects your left and right indicators together, causing all four indicators to flash, ie Hazard Warning. Just three connections to the wiring (located under your steering column), and a hole for the rocker switch. Comes with detailed fitting instructions and help line.

From the descrition I cannot tell if it will work wth the MGA 1500 with turn signal relay box. When refering to steering column switch, it appears to be designed for later models without the relay box.