Sharon, I am guessing you already have your RT Tickets. So what you need first is a connecting flight in London to some other location. I would look into that first. I am not a big train fan. Anything more than 4 hours and I would rather be on a plane if I can find a non-stop discount carrier. So when you look for that connection out of London, look for places that have discount service to Paris (as well as the train schedules).
The tool I use for this sort of thing is Google Flights. First put in Paris to Europe and filter with non-stop and one way. This will show you all the cities that have direct connection to Paris. Not ever discount carrier flies every route, every day. So if days are flexible, check a few days. These flights by the time you pay for luggage and all the other little charges will cost $100 to $150, even though they may say $65. Still a bargain. I will send a little more info to your PM.
Then once you have selected a few cities that are interesting you can do the same exercise out of London. London has a handful of airports and not many (any?) discount carriers fly out of Heathrow (I am just guessing that's where you are flying into). So you will either end up with slightly more expensive flight or have to change airports which is a pain.