> land at Heathrow you don't go pass passport control if you change onto another international flight. You go through security and then into the departure lounge.
This requires a Direct Airside Transit visa, unless you are a citizen of one of the exempt countries or have a visa/residence permit from certain countries (US, Canada, Australia, New Zealand, Ireland, Schengen). Applying for this visa involves going to the local visa processing centre, so it is not an online or on-arrival process, and takes around 3 weeks.