Curious WW2 ARP Lapel Badge With White Cross

I visited Abergavenny flea market today and found a number of home front badges which were all at remarkably low prices. It's rare these days to come across a badge I have never seen before but this small blue and red enamel lapel badge also had a white cross attached to the bottom. It has a pin brooch back. At this time I have no idea what this represents.

UPDATE: members from a badge forum thought the cross - a saltire - could be a good luck symbol or possibly related to the badge coming from Scotland.

Curious WW2 ARP Lapel Badge With White Cross