What do you think would happen if the U.K. were to be attacked?

This is all hypothetical of course but what do you think would happen? Say if Russia or China were to bomb us, do you think world war 3 would break out? And if so who do you think would be on our side? And who would be on there (the aggressor) side? I'd like to think it would be like WW1 & 2. With the Americans and French being on our side. But I'de like to here from you all, so cheers!