I used to live in Devon. Obeying the locals I made sure my broad beans
were planted on Boxing day. I have now moved to Margate, Kent. We are
five mins walking distance from the sea so it can never get too cold
here. Should I plant my broad beans on Boxing day here?

Up here in the 'grim north' most people plant by November - its so cold/dark December/January there is not much chance of anything growing outside!

Are you sowing or planting? Either way I'd protect with a cloche in case things get very cold/wet - a deep bed drains best when its really wet.