I used Parcel2go.com for sending a parcel (not international) selected parcelforce & dropped it off at a local post office,no hassle & at a god price
Try parcel monkey too.
If sending abroad i found Interparcel https://uk.interparcel.com/ to be one of the cheapest. I'm sending a parcel tomorrow from UK mainland to Austria 2kg 30cm x 30cm x 10cm £10.38 inc VAT whereas Parcelforce quoted more than £37 inc VAT (without the 14% discount) Cold from me
Poor service and very few offices . Wish it was part of royal mail as it gets parcels from abroad from royal mail counterparts .
15% off in the post office if you ask for rewards4u