Warning triangle, high vis vests (in the car, within the reach of the driver), set of bulbs are required (the bulbs aren't technically legally required, but if you have a blown bulb and get pulled over, you can't go anywhere until you've replaced it, so you need a set).
A fire extinguisher is also a nice thing to have. And confidence in your radiator & water pump. It got to 39 degrees when I was there last summer.
And yes, speak to your insurers. Not all standard policies cover use in Europe. There's also a firm listed on the MSE website that does europe wide breakdown cover for £70 or so, but they do state that the car should be inspected before you go out – ie, service it or MOT it.