I presume you mean you want to freeze food for your family in UK? In India, unless your family have an generator, it is not a good idea, as electricity supply is often interrupted several times a day, specially in summer. This is not good for frozen food. I will try to answer your questions one by one:
1. for how long can I freeze food. like can i freeze it for 1 month or so.
Ans. Yes you can. Food must be at room temperature before being put in the freezer.
2. which vegetables can i freeze? I was thinking of mix vegetables, matar paneer, palak paneer, black gram etc.
Ans. The vegetables you mention, freeze well. They must be defrosted fully and re-heated fully before consumption. Things like baingan bhurta, karela, kadhi, patra, pakoras, chapatties, parathas, poories also freeze well. Most vegetables will freeze, but they should be left slightly undercooked so they don't get too soggy whne re-heated. You can freeze some cooked masala mixes; like onions, tomatoes, spices cooked up to the stage where you would add vegetables. Freeze it in desired portions. Your family can then add it to any vegetable they want to eat/cook, for example tinned chickpeas, black eyed beans and fresh vegetables. Koftas can be frozen separately to their gravy and put together at the re-heating stage. Potatoes curries do not freeze well in my experience. They go a little crumbly.
3. do i need to add any preservatives?
4. what is the best method to freeze it? in ziplogs? in plastic containers?etc
Ans. Either will do, as long as they are clean. The things that have gravy, are better frozen without adding water. The person defrosting and eating them can do add water, when re-heating.
5. and how can i freeze rotis?
Ans. Do not butter them. If possible, do not add oil/ghee to the dough. Cool completely. Freeze in bags in small numbers, for example 5-10, one or two days portions only. They can be reheated on a hot griddle/tawa. If you coax them into ballooning up again, by pressing gently with a kitchen towel, they become almost like fresh.
The one tip I know of is to freeze in small packets so that u dont need to defreeze it everytime u have to eat it.
Ans. Exactly right!
It is going to be hard work cooking for a long time away. Try to encourage the person/s you are leaving all this food behind for(husband and children?), by leaving them simple recipes and spice mixes, ready to use. They might actually enjoy it! :-)
So where in India are you going? I am off there myself soon, but only for a short time.