Survivors in England assume that everyone in mainland Europe is dead. They no longer have sattelite technology to verify this, but they've gotten no radio signals and nobody who's ventured toward Calais by boat has spotted signs of life. Unbeknownst to them, there are survivors in Brittany. The nuclear blasts EMPed out enough equipment that the Brittany survivors can't simply make radio contact. They're ekeing out an increasing miserable existance, and somebody gets the bright idea to send up a signal in hopes of attracting attention from England. Ideally, I would like this to be something visible from the coast of Cornwall.
What would be involved? My first thought was 'fireworks', but nothing I've found tells how far away fireworks are visible. One fireworks display was described as 'visible for hundreds of miles', but it happened on top a mountain. I've found ways of getting things to the height of a mountain, if you have the right equipment - the Paris Gun was supposed to be able to launch shells 25 miles high, for example, and modern professional fireworks are apparently launched with something that closely resembled artillery. However, I cant imagine the surviviors having one of those lying around.
What's left? What makes a really big flash?