London to Mont St. Michel

Practical? Well, it depends on how you define that and how many days you are allocating for the trip. Going to Le Mont Saint Michel from London as a day trip? Probably not going to work out for you. To get to Le Mont from London, you could take the Eurostar to Gare du Nord (Paris) and then go across town to Gare Montparnasse and then do the approximately 4-hour train trip to Pontorson and then the bus to Le Mont. You'll have to look at the train schedules to see if you should arrive the night before to catch the first train or plan to spend the night in Normandy. Or maybe you could do one of the 14-hour day trip tours from Paris, but I don't know if Eurostar timetable works with that for a day trip. Another option is to take the Eurostar to Lille and take the train or drive from there. That trip will be about 6 hours by train and maybe 9 by car. Or you could see if there are flights between London and someplace close in Normandy or Brittany. I'd check Flybe for that. Caen pops to mind and it is only about 1 hour and 45 minutes from Le Mont.

Another option to go from Paris to Mont Saint Michel (maybe the quickest in public transport), is to take a TGV train from Paris Montparnasse to Rennes and then a bus to Mont St Michel (Illenoo bus). You find some hints how to reach Mont Saint Michel on http://visite.bretagne.free.fr/index.php/en/nature/mont-saint-michel.html

Are there any ferries that get you from England to close by? We thought about looking into that but then changed plans.

This is a somewhat romanticized view of an obsolete travel situation. The ferries are slow and often a rough-crossing. London is not itself a port, and not that close to the ferry ports. This is not like going to Disneyland and considering whether it's worth it to see the San Diego Zoo on the same trip.

While they are not remotely similar destinations, I would encourage you to consider fascinating medieval places in the U.K., like York, or castles or abbeys in Wales, Scotland, and so on, before spending a very long time getting to MSM from London. A halfway measure could be some ruined abbeys or even Stonehenge (a lot older ... !) from London. Some of these are even Harry Potter film locations, like Lacock. While I like the major cities of the world, you could easily spend more time seeing the most southerly part of England as you could spend seeing London itself.

Unfortunately, due to Henry VIII, many of England's older abbeys are in ruins. MSM is in great shape.

It's definitely NOT a day trip, if that's what you're wondering. One "out-of-the-box" idea would require at least one night away and a bit of planning, but we did this a few years ago and it was quite memorable:

We took a train from London to Portsmouth and caught an overnight ferry to St. Malo. The best part was the party atmosphere on the boat, as everyone gathered in the theater to watch Spain beat Italy in the Euros. We had a berth on the ferry, and arrived in Brittany early the next morning rested up and ready to drive the rental car we picked up at the ferry dock to Mont St. Michel, about an hour away. We had plenty of time to walk the island and have lunch, and even detoured to Juno Beach before driving to Paris, where we dropped the car that evening.

Depending on whether or not you need to return to London and how much time you have after your visit to Mont St. Michel that afternoon/evening, you could either drive to a train station, drop the car and catch a train to your next city/EuroStar back to London, or drive to an airport to catch a flight to wherever you need to be.

Mont St. Michel is beautiful. But so are the surrounding provinces of Normandy/Brittany. As others have correctly noted, this is certainly not a realistic day trip from anywhere in the UK (not even close). I would go further and suggest that if you really want to see Mont St Michel, you owe it to yourself to take at the very least few days to do the trip, so you can also see a bit of Normany and/or Brittany in addition to spending a night on Mont St Michel (don't just go to Mont St Michel for a couple hours in the middle of the day, that's the worst time/way to go). Normandy and Brittany have a lot to offer in addition to Mont St Michel - they're wonderful.