From the Heathrow Airport Website
Please take note of
Connection times: important information
Pages within this section may quote minimum connection times. These only apply if you have booked a through-ticket with an airline or travel agent, and any baggage is checked through to your final destination.
Minimum connection times do not apply if:
* You have arranged your own flight connection; or
* You need to pick up baggage at Heathrow
In these cases you will need to follow signs for arrivals, then check in as normal for your connecting flight. You should allow extra time for your journey. If you need to change terminals, please visit our travel between terminals page.
Extra time being the time it takes to go through immigration, collect luggage, pass through customs, get to departures, stand in a queue, check in, queue for security...
... which would realistically mean adding 3-4 hours to the connection time depending on where you are going and with whom.