it would largely depend on the amount as the largest value postal order is £20 and there would be a fee for each one.

Postal Order fees are here:
http://www.postoffice.co.uk/portal/po/jump1?catId=86500737&mediaId=73500709

I can't find a fees page on WU, but you can contact them here to find out:
https://www.westernunion.co.uk/WUCOMWEB/staticMid.do?method=load&pagename=he...

Depends on where you want to send money too.
For Europe, the money is received immediately, other countrys have either 12hours or 24 hours delay in receipt

Charges in Europe and most african contrys vary in amounts starting from £4.90 for up to £100

If you let me know how much and where yo want to send it to I could give you the rate.

You can find out here;

http://www2.ukmoneytransfer.com/PriceIt.asp

Although as stated above, it depends on where you're sending the money to.

If you're sending the money within the UK, Postal Orders, or a Banker's Draft, will be cheaper