We are traveling to the UK and France from March 19 thru April 12. We have purchased Brit rail passes (exorbitantly expensive @ $1,200) and London tube passes as they were cheaper than buying once we got there. But in France April 3 through April 10, and besides the Eurostar from London, we are looking at 4 days in Paris, and taking the train to St Malo and staying the other 4 days. Is it better to purchase train tickets in advance? Or wait until we arrive and are ready to travel? I looked into a ferry from St Malo to Geurnsey and then back to the UK and London, but it looks like the ferry only runs a couple of days a week, so we would take the train back again to Paris and the Eurostar back to London to get on our flight back. (Next day) Also, I am experiencing some shock at how much we have already spent just on travel and accommodations. (approx $2400 for accommodations, a myraid of Air B&Bs average 50-60 per night and hotels anywhere from 100 Euros - 180 Euros per night, then train and tube passes $1200 for UK and plane tickets $2600) this is not including any meals and/or entrance fees to anywhere we want to see. I know it would be expensive, but now I am wondering how we are going to eat while we are on vacation (sarcasm) LOL. Please advise on any further ideas and whether I should buy the train tickets in advance. Thanks!
Train tickets from St-Malo to Paris do vary in price and if you are looking to save money, you would be advised to buy them in advance. Of course, the usual caveat holds that cheaper tickets are inflexible, i.e. they are good for that train and you can't modify or get them refunded, and they're worthless if you miss your original train.
My usual advice is to go on the SNCF or Trainline website and do some dummy bookings for Paris and St Malo tickets for a couple of days out and see if you're willing to wait to pay those prices if you wait until the last minute.
But honestly, in this case, since you already seem to know your dates and have a fixed itinerary, I would go ahead and just buy the tickets now. They certainly won't get any cheaper as you get closer to the date.
Have you purchased your Eurostar tickets yet? Those will also only get more expensive as it gets closer to the date of travel.
With regards to food, Marks and Spencer will be your friend in England.
And Monoprix in Paris -- or just consider eating in Paris part of the sightseeing experience.