An A320 sim would be a challenge for someone with no airbus experience, and a Boeing sim would be the same for an airbus only pilot. The BA sims are, as suggested above, looking for teamwork and soft skills as much as they are physical flying ability. They're also, crucially, looking for an upward trend in your ability over the course of the assessment - do you get better throughout the sim, and make the same mistakes less etc.
I think it's entirely reasonable, and actually very sensible, to do a practice sim before an assessment. It doesn't have to be a 787. If you are an airbus pilot, simply going in a 737 sim would be enough to get you used to having a yoke and having to trim. that alone could be the difference between having enough capacity to complete the sim successfully.