Update:
as long as you dress smart, you don't need to where suite and tie, but remember that first impressions do count!
Process for London:
Initial Registration - Document checks, Make sure your passport is Valid, or take FULL birth certificate. Bank Statement in last Month dated.
Interview (maybe group of 2) :
Sight Test - can you read the letters on the sheet (basic opticians test)
Smell Test - identify the liquid in a cup.. (used for sniffing out drunks, petrol.. stuff like that at the games)
Conflict History - explain a time when you resolved a conflict, and what actions you took.
Photograph - Your photo is taken
Document Check (Final) - you will finish your application here, walk through your application form and fill in the blanks.
Finished!
Background Check - this can take upto 12 Weeks.
Overall Time at Interview Day - 3-4 Hours (Based on London where it was packed and the queue was big!)
Good Luck!