To send a Xantia radio, which is just a bit too large to be a Royal Mail 'small' parcel, the cheapest my local Post Office (very obliging) could do it for was £8
However, I discovered that Collect+ (book and pay on-line 24/7, leave the parcel at a local corner shop) will deliver anything up to 2Kg for £3.99 (the radio packed weighs a bit less than 1.8Kg) anywhere in mainland UK. 2-5Kg for £4.99, 5-10Kg £6.99. If it's more convenient for the recipient to collect the parcel from their nearest 'corner shop' agent, Collect+ will leave it there for collection and will let them know by e-mail when it's ready for collection. The agencies are all early 'til late small shops so parcels can be dropped off or collected whenever the shops are open. Collect+'s website is really well thought out: you just stuff in your own postcode and the map shows you the nearest agencies, you pick one, stuff in the destination postcode and it shows that together with the nearest agencies to that. Payment by PayPal or any Credit/Debit Card (except Amex od course). Simple idea, well organised and implemented - wish I'd thought of it!