You can not get from Terminal 3 to Terminal 1 in under 45 min. I had an unexpected night in London because of this. I arrived in Terminal 3 with 45 min until my flight to Armenia took off. However by the time i got to the check in desk there was 10 min and the man wouldnt let me on. It must be the only time in the history of the London-Yerevan flight that it has been ontime.
Yesterday when i did get to the check-in desk on time we were delayed over an hour while they changed planes as there was a problem with our orginal one. I would have made that flight!