Didn’t Lizzie miss an opportunity to make it up with Lily by taking her out to celebrate her results?
She offered to take her out before the Russ affair came out, and at that point, Lily said no.
I.think Lizzie is treating Lly the way she is because of how Lily was last week, barely saying a civil word, and Lizzie doesn't want to go back to that.
Also, she will probably be remembering her own relationship with Robin, which will be fresh in her mind, as he was just back in the village. So she'll also be wanting to keep Lily talking to her so she does know what's going on (even though we know she doesn't know all of it.) And with the Robin history, she might feel hypocritical if she has a go at Lily about it.
Though I suspect keeping the lines of communication open wouldn't have gone as far as inviting Russ round for supper, if Lily had given her a choice.
I also think Tom wanted to show Natasha his sausage, but I think he doesn't stand a chance in he'll. Until the scriptwriters put in a personality change for her. I'm looking forward to hearing her feedback on the farm, though.