Hi there.
I have just been for an interview with BMI for EMA/BHX, and there was no written exam.
I'm not sure if it is different for LHR. We watched a short presentation on the company, then were split into groups. We had to make a paper animal, then had to do some short role plays, whilst being assessed. That was it!
We then left the room for approx. 40mins, and were then all called back in. We had all been successful! I had my 2-1 interview, which was fine, and have just been offered the job.
I would say don't worry too much about it, just be yourself, and you will be fine xxx