The 48 hours is just a safety thing for them really, so that they don't have to worry about you turning up before the guy's got back with the van and demanding that your parcel should be ready for pickup immediately. The vast majority of the time you shouldn't have any problem getting it earlier, I normally go to the post office the next day just to avoid finding out that it's still out on the van but you can always try going a bit earlier if you don't mind being possibly told it's not back yet.