Looks impressive on that map. I don't think that I've ever seen a single Met Office warning like that covering the vast majority of the UK in this fashion before.
I think that is because the risk area is big, but the potential in any one area is not necessarily all that great- hence the low risk/moderate impact matrix. With storms it is always difficult to be more precise until things actually get going. Many people in that warning area will see nothing at all - that's the nature of these things.
The warning area itself may well be refined later on today.
Ben,
Nr. Easingwold, North Yorkshire
30m asl