We allocate the courier tracking code once the sale is checked. It can still take 24 hours for the Courier to pick up the parcel. Depending on the tracking scanner the infomation might not get added to the database as "picked up" until the driver does his upload.
On rare occasions a driver can miss scanning a parcel. But they try not to do that because they do not get paid without scanning it.
Goods are first shipped in bulk to the UK by Air, then customs and straight to the Royal Mail sorting area. This can take a few days and no scans are taken until sorting at Royal Mail begins.
So do not worry if the scan does not turn up. Quite often the parcel will turn up on the door before you get a scan result.
This message, "We have been notified that your item is being shipped by the retailer; the next scan will appear when we receive this item.", this means it is still being shipped to the UK.