I also would like to know if they allow you to go into duty free in goa, does anyone know?

I have never been to an airport where you could so I would doubt that you can in Goa. The duty free is always situated in the departures area. They want you to pay tax in the country you have arrived in , not avoid it, however, on the way out they will happily try to tempt you with one last effort to part with some cash.
Manchester airport has a shop as you leave the baggage collection area and after you have passed through customs. Don't be fooled into thinking the prices are duty free because they are not.
As for buying duty free for consumption in Goa, as previously mentioned it's cheaper to buy the likes of Gin and Vodka in the local store. I was paying about £3-£4 for a 70cl Gilbeys gin. If you want special liquers it might be best to purchase them in duty free GB as they may have limited choice when you try to buy in Goa but as already pointed out you would have to pack them into the hold baggage if you are transferring planes at Mumbai. You couldn't purchase them in Mumbai for the same reason you can't in Goa.
Must admit, flying Kingfisher seems to offer the opportunity to stuff a few bottles in the luggage on the way home but if, like us, your airline only gives you 20kg of luggage allowance and an excess charge of £33 per kilo, it makes taking bottles home a non starter !