Well done on getting an interview, can't fault your choice of Company.
The interview process seems to have changed a lot since I started, the company is a lot bigger, employing a lot more staff, so things are more structured now.
Chalet Hosts/Assistants used to be asked to take something like a cake with them to the interview, but I don't know if they still are...it'll tell you in the details that you get sent prior to your interview exactly what you need to take.
If you do have to take a cake, when you're asked about it, make a point of telling them you the processes you used in baking it. They're not just interested in end results, they want to see that you know how you got there. For all they know, your Mum made it for you. Something worth mentioning for 'bonus' points...cake recipes that work at home won't work at altitude, they don't rise...this is why the majority of cakes in ski chalets use yoghurt as an ingredient.
I know they definitely do group exercises now, so be prepared to get involved, but don't come across as over-bearing/unwilling to listen to others. Give your opinion, listen to others.
If you do well enough in group interview, you'll go onto a 1on1 interview. Most of the interviewers are the Management team from the various resorts and the majority of them started in a job like you're applying for, so although they know the job inside out, they've been in the same position as you . Do a bit of research into the company...what 3 brands do Hotelplan UK operate, what countries/resorts do they operate in etc. Be prepared to be asked what you expect to get out of the season...and don't tell them you want 5 months on the piss!
Good luck, and if it all goes well, I'll see you in Les Arcs for your training...I'm one of the Training Chefs for next season.