I think I would plan on about 3 days in London and then a bus tour swing through the heart – literally and literarily – of England; down through Winchester, Salisbury and Stomehenge, then across to Bath and northeast to Oxford and Stratford-on-Avon. You should be able to do that in about 4/5 days if you rush. The Underground is great for getting around London but if you are afraid of losing kids en route, you might want to contract with a tour guide and bus company for that part as well. If you find a hotel that provides breakfast, you will save some on food which costs a lot in London. If you have more questions, I would be happy to answer as best as I can. I am American but spend a lot of time in England. Many places like Stratford will be overrun with tourists at that time of year but if you want to hit highlights, you’ll have to put up with it a bit.