Based on last year, APL are likely to send start dates around one month after we were notified of our success.
I was asked at interview about my availability, and advised of my 3-month notice period with work, which would take me to August/September, but that I'd prefer October. However, I will be happy with whatever I am given, sooner being better as seniority is king at BA.
Whilst I'm here, to answer another question: I was at OAA on 23rd March and Waterside on 23rd April and 7th May, both times in the afternoon. In all cases, I was "the old guy" in the room