You can take bottles purchased in duty free on board both in Britain and abroad.
True, but beware if you are travelling on onto a connecting flight, the second flight will not let you through security wiith duty free bottles from a previous flight. You have to add them to checked baggage at the connecting airport.
Example: If you go Heathrow to Goa via Mumbai, you can buy duty free bottles at Heathrow and take them onboard, but you cannot take them as hand baggage through security at Mumbai onto the internal flight down to Goa. You can either put them into your checked bags or check in an extra bag.