Definitely ask about travel expenses, a lot of companies will be willing to reimburse. I've heard of some places doing Skype interviews (provided there aren't any group activities). I just recently had to cancel an assessment centre due to travel. It was at the other end of the UK and had multiple rounds. I wasn't confident enough that I would get it to pay £170 - £200 per trip which of course is a bit much to be reimbursed for.