The courier/delivery company varies between products and suppliers. 

As an example, for small/medium goods, your order may be delivered by Royal Mail, Yodel, DX, APC, FedEx, UPS, EVRI and DHL. 
For large/heavy goods, your order may be delivered as a pallet /2-man delivery with companies such as BJS, TPN Connect, Parcelforce and directly by the manufacturer/supplier.