If they have the parts in stock and aren't busy, it may not take longer than couple hours. If they have to wait for the screen to arrive or are extremely busy, it takes longer. Ask them, they know better than any of us do
Two weeks is about right. My old Santa Rosa MacBook Pro broke (GPU failed, and screen blew) and turn around was 2-3 weeks. Though that was during then 2nd year of warranty.
1st year of John Lewis warranty is the normal 1-year AppleCare. 2nd year of warranty is through John Lewis and they send it off to Cancom for repair.
Depending if your laptop is in its first or second year of warranty will determine who works on the repair.. and if it is Cancom, it depends if they have the part in stock.