He won't be allowed to board the BRU-LHR flight.
He needs a valid entry clearance for the UK. The Indian citizen may still opt to approach the UK border at LHR and claim asylum. In that case the airline, which brought him to LHR, will be facing a heavy fine.
There are some exceptions for Indian having an airside connection and certain visa types for USA, Canada, Schengen etc. These expection do not apply here.