Check the below links for more info on Transit via UK.
http://www.britainusa.com/visas/visas.asp
http://www.britainusa.com/consular/other_show.asp?SarticleType=25&Other_ID=597
http://www.britainusa.com/visas/visas.asp
http://www.britainusa.com/consular/other_show.asp?SarticleType=25&Other_ID=597