A friendly word of advice - try to avoid Heathrow! Connections through London's airport will set you back a minimum of 2 hours because of all the security measures in place at Heathrow's terminals.
Many
alternative routes via other Uk airports are available, not to mention the possibility of flying to Dublin and taking a bus from the airport to Belfast Great Victoria Street Station, located just behind the Europa hotel (information on bus links between Dublin and Belfast)