I found out recently that Royal Mail provide a unique reference number when you request for free certificate of posting for small items. This reference number will be provided for first or second class parcels to the UK.
eBay will accept this as proof of delivery, this avoids paying an extra £1 for tracking.
There is still some risk attached as standard post only covers £20, this is not fully tracked and is only updated online when item has been delivered.
Certainly worth it and can deter fraudsters on eBay who falsely claim for items not received.