Hi,
The interview itself is similar to that of BA (being a franchise company.) The maths test involves a lot of currency converting (converter tables are supplied.) You're not allowed a calculator, so for someone like me it meant a lot of counting on my hand!!
Part of the test is an English test, there is also a section about GB Airways - all the information is mentioned in a presentation prior to the test, so make mental notes!!
The 2:1 interview (if you get through) asks questions about experiences you put on your application form, - think of other experiences!!!! an example question: "Tell me about a time you worked well within a team apart from the time you stated on your form."
Hope this helps