If you're getting the contracted speeds from the hub, VM have fulfilled their contract - how you connect your equipment is not their responsibility, and the minimum speed guarantee only applies to a device connected to the hub directly by ethernet cable. Chances are that the powerline network plugs are connected to different electrical circuits, because that often impairs the performance. Simplifying the many options open to you, your options are:
1) Run an ethernet cable from hub to upstairs PC; Cheapest and most reliable solution, often very tricky to implement neatly.
2) Pay VM £99 to move the hub closer to the PC; Costly, may worsen downstairs wifi, no benefit if you change ISP or move.
3) Buy a mesh wifi system (around £100) and put the hub in modem mode, and have the upstairs PC connect to one of the mesh wifi units; Costly, but effective, improves wireless around the property, still useful if you move or swap ISP.