The Gander stopover was for performance issues on the dept and climbout from LHR due to the heat.
Crew had a fuel plan for both but Gander always looked likely as the a/c was BA heaviest (G-BOAC) and it was full with 100 pax.
The BA001 the following day was delayed by 90mins to allow the temp to drop down to around 27/28C, so that a stop would not be required.