I was looking to guarantee a couple of big Xmas packages would arrive before Xmas, and found the Post Office somewhat lacking. After a little on-line research at MSE I found this deal, which amazed me.......
Basically you can use "parcelmonkey" as an on-line agent to book a next-day courier service. The couriers are, apparently, DHL, Fedex, City Link, etc. You can send packages on their "next day before 6pm" service, up to 10kg, with £50 insurance for just £6+vat (i.e. £6.90)! Additional insurance is available for valuable items. You can also get £2 of Quidco or topcashback on your first order. This effectively makes a one-off deal of £4.90! CRACKING!!
You do all the booking on-line and then pay for the service. You then print off a label which you stick to your parcel. The courier will then come and collect the item and deliver it the next day. You can even specify a work or neighbour address for collection if that helps.
p.s. credit to robot1000 who posted this 5 weeks ago. I hadn't registered that deal at the time and I hope that reposting it now helps someone else this week with their Xmas pressies!
EDIT - eldudereno (and others) have suggested using http://www.parcelbee.co.uk
as a slightly cheaper alternative, especially for big packages - i.e. £6.49 for up to 30kg. No Quidco or topcashback though....
be warned, Parcelbee seem to be going under. see here http://forums.ebay.co.uk/thread.jspa?threadID=1100290973&tstart=0&mod=1267474832158