Thanks for all your replies. I just thought car hire may be really expensive, will look into it. Don't know how my aussie kids will cope with the temperature, they are use to living in their shorts and thongs. They don't even own any long pants yet so will definitely need to do some clothes shopping.
They are so excited about going to the UK, they want to see the Christmas lights, visit Hamleys. Just got their British passports and was very impressed with the speed I received them exactly 2 weeks from the date I sent them, so that time included postage. Shame we will miss fireworks night as that is the date we leave Australia. Two of my kids are hoping to come back with British accents lol. I have always ensured that they are proud of their british heritage, however they can't quite support England over Australia in the cricket.