I reckon like others have posted, need it overnight so engine is cold for valve clearance check, which is required at 16,000 miles, hence 2 days. Unless you can drop it off night before the service date. It shouldn't cost more, just because they need it for two days. Your only paying for the 16,000 mile service, however much that costs, unfortunately, it's a major service as it involves the valve clearance check.
Wouldn't worry about the loan bike insurance, you should be covered on their insurance as its a pool vehicle.