I want to go from london to paris and back through eurostar and have some questions:
1 . how early do i need to book my train tickets .. i will be travelling in may and even though the booking is open , would like to know when and how the prices will shoot up and whether i have time.
2. its really crazy that they all want us to buy a non refundable ticket as the rest (semi flexible,flexible) are outright expensive. Is this the only option that i have . Right now i have tickets reasonably placed but am wondering whether thats the only choice i have.
3. How much time does it take through immigration in both london and paris , meaning how much earlier do i need to come to the train station . say if i want to take the train at st.pancras station in london how early do i need to be there to go through customs and immigration.
4. I will be travelling with a 2 year old baby ? What kind of food can i get within the train?

"<i>1 . how early do i need to book my train tickets .. i will be travelling in may and even though the booking is open , would like to know when and how the prices will shoot up and whether i have time.</i>"

Book as soon as your dates are firm -- the longer you wait the higher the fares.

"<i>2. its really crazy that they all want us to buy a non refundable ticket as the rest (semi flexible,flexible) are outright expensive. Is this the only option that i have . Right now i have tickets reasonably placed but am wondering whether thats the only choice i have.</i>"

It is just like airlines or hotels -- the more restrictions the cheaper it is. If you want fully flexible tickets you will pay more.

"<i>3. How much time does it take through immigration in both london and paris , meaning how much earlier do i need to come to the train station . say if i want to take the train at st.pancras station in london how early do i need to be there to go through customs and immigration.</i>"

You want to be at St Pancras at least 45 minutes before the train's departure. You go through immigration there and then just walk off the train in Paris--no additional immigration formalities. For your return trip it is the same - be at Gare du Nord at least 45 minutes early.

"<i>4. I will be travelling with a 2 year old baby ? What kind of food can i get within the train?</i>"

The trip is only 2+ hours so you can easily carry any special snacks/treats your child likes. But food/snacks/drinks are available on board. If you travel standard (cheaper tix) you don't get a meal -- but there are sandwiches, pastries, some hot food, etc.

Food options in the buffet are a bit limited and not inspiring. Here's one of their sample menus, but the food I've seen on board always struck me as being a bit heavy on pastry and sugar:

http://www.eurostar.com/pdf/food_on_...class_food.pdf

Also, you have to make your way to the buffet car, which might be one way to pass some time for your toddler, but might not be such fun for you.

Inside St Pancras station there's a proper Marks & Spencer food shop, and a mini-mall called Sourced Market with a number of fresh food retailers offering a good selection of real food. As well as the wall-to-wall sellers of oversugared junk all major British stations are stuffed with. There's usually no restriction on what food can be taken from Britain to France.

Bear in mind though that the check in deadline (explained on your ticket) is mostly defined by the time you put your ticket through the gate leading to security and French immigration. Beyond that gate, you've got access only to muck.

To get to the decent food stores in the station you need a couple of minutes walk, plus the time spent at their (often backed up, but always fast-moving) checkouts.

<< Is this the only option that i have . Right now i have tickets reasonably placed but am wondering whether thats the only choice i have.>>
You could also use Euroline coaches. They might be cheaper somewhat and it is a nice option in May for the ferry ride. Check: http://www.eurolines.com/

Bus and ferry is a long ride with a child.