Jyothi ,
There are range of things that you can buy but some of the stuff I normally buy when I travel to India is
1) Fridge magnets , souvenirs , show pieces for friends.
2) Perfumes & shower kits, winter wear for relatives and cousins.
3) You can also get ipods & digi cams for a reasonable price nowadays if you want to gift.
4) Handbags , purses for ladies.
I can't think of anything specific which you find only in London and cannot be found in India because all the big brands are available everywhere and remember most of the stuff you buy in London is invariably labelled 'Made in China' or 'Made in Thailand'.
Hope you have a nice time shopping

.
-Sabitha