You'll arrive into T2 and then have to connect to T1 through Flight Connections. You walk between T2 and T1 - it can be a fairly long walk too!
more information from the links on this BAA page
http://www.heathrowairport.com/porta...00147e120a____
some info from BD here
http://www.flybmi.com/bmi/en-gb/trav...rport=LHR#info