I have a J ticket in class I booked on Sunday from LHR to DME. I have been trying to upgrade using Avios but the message I get says that BA cannot upgrade my flight using Avios. Is this because of no availability or because I class is not eligible?

For what it's worth, Expertflyer shows the following for my flight:

F9 A9 J7 C5 D3 R1 W1 E1 Y0 B0 H0 K0 M0 G0

So the back looks oversold. As a silver should I holdout for a possible upgrade to F at the gate?