Hmmm, so you're thinking of modelling rotten keys?
The answer to that one is, no , gangers would not let a key get into that state on the main line. Don't forget, you are looking at a siding, and oak keys are pretty scarce these days, I don't know if Didcot has the machine to make them. Try and get hold of "Track Topics" the GWR book, it'll tell you about the mechanisation at the track factory.
Are you really sure you want to model to the degree of detail you are proposing? If so, 4mm would be way to small, consider going to G1, cobwebs become nearly visible. as a gauge, fine for individual models, but for capturing a slice of the railway - maybe not.