I'm tempted to simply write "Wear clothes. Catch train to Yorkshire" - but I suspect that you'd prefer a more serious answer.
I note that your question doesn't say 'cheapest'. If cost isn't a major issue then Royal Mail's Special Delivery service (available at any post office) is probably best. Packages are handled separately to the general mail, tracked and signed for. The price of sending a parcel weighing up to 2kg is £11.00 but it then shoots up to £25.80 for a parcel weighing up to 10kg. (So, where possible, it best to split a package weighing between 2kg and 4kg into two packages of less than 2kg each).
Those prices are for guaranteed delivery by 1pm on the (working) day after posting. If the package is really urgent there's a premium service that guarantees delivery by 9am. It costs £26.16 but it's only available for packages up to 2kg. However Parcelforce Worldwide also offers express delivery services. For example, a 3kg package costs £40.92 for delivery by 9am on the next working day, £30.92 if it can wait an hour longer until 10am or £20.92 for delivery by noon.
Once you've weighed your parcel (or, at least, estimated its weight) you can find prices for all available services (from both Royal Mail and Parcelforce Worldwide) here:
http://www.royalmail.com/price-finder
(Note that some prices need to have VAT added onto them. Multiply the price shown by 1.2).
Chris