It sounds like you’ve done an excellent job keeping these students motivated throughout the course.
As most of their energy is now spent preparing for their time in the UK, you could work on this in the lessons. Brainstorm with the students a list of situations they’re likely to encounter during their first week. For example:
Meeting their new colleagues
Taking the bus
Asking for directions
Dealing with (lost) luggage at the airport
The list is endless. Ask them if there are other things they’d like to cover. Then you work on this language with lots of listening, speaking and role play.
You could also cover communication strategies. For example:
a) how to check understanding (I think what you’re saying is……)
b) asking for repetition (could you say that again, I didn’t quite catch it)
c) what to do when you don’t know a lexical item (e.g. I’m looking for something with wheels you use for carrying heavy things). You can take in pictures of unusual objects (or the real thing) and have students describe them.
It also sounds as if the students have got complacent, thinking they’ll work it out when they’re there. You could now start speaking to them as you would to a native speaker, i.e. don’t make allowances for the fact that they’re learning. You could also do a listening lesson with a cassette at a slightly higher level than usual. With this, they’ll realise that there is still room for improvement. I wouldn’t do too much of this as you don’t want them leaving the lesson feeling demoralised.
Of course you can also have a party in class as students will need to practise language for social situations.