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 it. Payment by PayPal or any Credit/Debit Card (except Amex of course). Simple idea, well organised and implemented - wish I'd thought of it!
Update:When it came to sending the radio, I discovered that Collect+ do in fact have maximum parcel dimensions but they're quite generous (small = 30 x 30 x 50 cm IIRC) so I was able to abandon the hunt for the smallest possible box that the radio would fit in and use one kindly supplied by Amazon with lots of space for protective bubble-wrap.
I dropped off the parcel at 8:50 on my way to work on a Saturday morning, getting a receipt with a tracking number in return (confirmed by e-mail). The consignee was alerted by e-mail when the parcel was waiting for collection at his local 'corner shop' and I received another at 11:24 on the Tuesday morning to tell me it had been collected. Not only was collection more convenient for the consignee, but it reduced the charge from £3.99 to £3.49. So "3-5 days" was in fact 2 working days - pretty good.
In complete contrast, when I got home on Saturday afternoon, there was a Royal Mail card to tell me that a 'Signed For' letter was waiting for collection at their local 'Collection Office' (open 7am to 12noon Monday to Saturday). Fortunately for me, it's 'only' a 4-mile round trip and there was no excess to pay.