The cost of the assessment day is £190, that's before any extras e.g. overnight accomodation. An expensive day out all in all, but I think the high price would never warrant turning down a potentially good opportunity.
As Sciolistes said, I think the best plan of action is to go, give it your all and then re-assess once you have all the information from ctc, i.e. who they're planning to ship you off to.