According to Comreg's report yesterday, only 78% of national mail arrives within one day, the target is 94%. I'd imagine you'd be pretty lucky for it to get to the UK within a day. I remember a few years back the company I was working for at the time sent the artwork for an add to one of the UK PC magazines via SDS guaranteed next day delivery, and later that day posted the cheque via standard mail. The cheque got there a day before the artwork (which missed the print deadline!)
Leo