For a random sample size and weight combination I tried, ParcelMonkey is offering just ParcelForce and DPD at reasonable prices:
https://www.parcelmonkey.co.uk
Parcel2Go is offering good prices though from ParcelForce, DPD, Hermes and Landmark Global, with other carriers available if you're prepared to pay more:
https://www.parcel2go.com/
Have a look in the supermarkets to see if they have any boxes you could use. I sent a 10kg parcel using two empty vegetable trays taped together and then wrapped up.
The trays were much thicker and sturdier than a cardboard box.