This sounds like such a great trip! We're taking our kids - 3, 7, and 9 - to this area in May! We've been traveling with our kids since our oldest was an infant and you are going to love it!! You experience a place in such a different way when you have little people with you.
I have three thoughts for you.
First, I feel you are asking a lot of your kids with your plan to travel from Cornwall to London and then an overseas flight on your final day. You are looking at a long car ride, followed by a train ride, followed by a long flight. Can they really sit still and behave that long? You know your kids best but I suggest thinking about how likely they are to be patient in the airport and sit still on the plane after sitting still so much of the day beforehand. We have always planned something relatively active before our international flights and stayed within 1 hour or less of the airport to minimize the amount of time they are in the car.
Second, a week or two before you depart start talking to them about what to expect at the airport - tell them the times they will have to wait patiently (ie. security lineups), that they will have to stay close to you all the time, that there is no fighting/yelling in the airport, that they will have to go to sleep at such-and-such a time on the plane, that they will be tired when you arrive but they just have to be patient until you get to the train .... whatever it is you expect from them. Walk them through the steps you need to go through to get on the plane - tickets, check baggage, security, wait, lineup to board plane, etc. Discuss it in a fun way every day, making it part of the excitement of the trip. The more they know what to expect, the better they will behave and easier your trip will be .... especially when you are all jet lagged and trying to figure out where to go in London.
Finally, I strongly encourage you to have each kid complete an art journal for the trip. I started doing this with our kids when they each turned 3. We just use a medium-sized multimedia art book with blank pages. Before we leave, we colour or paint a cover page and fill the first few pages with a map (either drawn or glued in), a page where the kids make a bucket list for the trip (I think ice cream is on every bucket list we have ever created!!) and a calendar which we fill in with our itinerary. Then, each day we fill a few pages with bits of things from the day (napkins, ticket stubs, post cards, a peacock feather my daughter found once, cut up brochures, etc... I pack a few small glue sticks), we talk about what they did that day and they draw or colour a picture about something they enjoyed (just scribbles when they are 3, so I label them), and I write one or two sentences about what they want to remember from the day (as the kids get older they have started doing the writing themselves). We also use colourful washi tape to stick in several photos of them doing something from that day (use an instax polaroid cameras or HP Sprocket). I hit up the scrapbooking section at our local craft store for some relevant stickers to finish off the pages.
My kids treasure these journals. They love showing them to their grandparents when they get home and it is a special treat when they are allowed to get them out (I keep them out of reach to protect them) and look through them. The best part is that they help the kids remember so much more about our trips. I can't recommend doing this enough!
Have fun!!! You are going to love traveling with your kids!!