AMBASSADORS come and ambassadors go, but Laxmi Mal Singhvi, outgoing Indian high commissioner in London, will leave behind impressions that in polite diplomatese can only be termed 'unusual'. In town and village, in city and countryside, in universities and town halls, in parks and offices, England is saying: Singhvi Was Here. The Indian high commissioner, who two years ago had publicly welcomed a Labour resolution that spoke of Kashmir as an area separate from India and which supported a plebiscite, is now busy with pursuits of a different kind. 'His Excellency Dr L.M. Singhvi' is inscribing his name in some of the most public places in Britain.