An 8kg turkey takes about a day and a half to defrost at room temperature, so it probably takes around twice that in a fridge. Thereafter though, the people who produce 'bootiful' turkeys say that it should be cooked within 24 hours:
http://www.bernardmatthews.com/christmas_defrosting.aspx
So buying it around 4 days in advance might be sensible but I doubt that 5 days would be a problem.
8kg? OMG, who are you inviting round for Chrissie dinner,? The Dagenham Girl Pipers? That's a LOT of turkey leftovers for sarnies, curries, dog food etc.
Enjoy your Christmas dinner hammerman. Happy Christmas xx