what was the significance of the pork pie?

I surmise after thinking about your good question that John felt that in some way he was paying his last respects to his deceased client by enjoying something Billy Stoke was previously associated with at the factory. He was told by the Asian man how Billy Stoke defiantly urinated in the vat of batter before his departure and that some of the batch went into circulation. I waited for John to smell that pie on the train before ingesting any of it and he did (though maybe to appreciate the aroma as opposed to being cautious since this was a very fresh pie!). He was so lonely he seemed to absorb a little of those lives of those he was trying to bury with some dignity.

That's my interpretation anyway 

"These days you have to boil someone before you can sleep with them"

Excellent interpretation sweetie

It's a sad and poignant point beautifully articulated by a superb film.