@Jumpseat
Do you mean for the final stages of BA or? If so then the Crown Plaza Heathrow is about a 10 minute taxi ride away. I do believe there are buses and BA will give you information on the schedules nearer the time should you be successful in reaching the final stages.
@Warp
Based on BA's website, you can expect numerical reasoning, verbal reasoning and some aptitude testing at the first BA stage and then just a final stage interview. The reason for BA splitting the final stage into two this year is because it makes sense from a HR point of view as lots of candidates in the previous years were not meeting the required level in either the verbal reasoning, numerical reasoning or aptitude tests. This year HR will only be interviewing the candidates that have passed all these tests and thus it will save a lot of HR's time and resources.