downton abbey - Was Thomas' hand injury in the trenches impulsive or premeditated?

I'm not sure why the Wikipedia synopsis as you quoted it puts a link between Matthew’s final words during their conversation (“War has a way of distinguishing [...]”) and Thomas then getting himself wounded. I think the link lies clearly in the exchange they have just before those words:

Thomas: Miss O'Brien keeps me informed. Lady Edith's driving. Lady Sybil's training as a nurse. Miss O'Brien tells me the hospital's busier than ever with the wounded coming in. That true?
Matthew: Certainly is. They had a concert when I was there to raise extra funds.
Thomas: I'm curious, sir. Do you think I could ever get a transfer back to the hospital, seeing as it's war work?
Matthew: Well, you'd have to be sent home from the front first. And then you might have to pull a few strings.

Source: complete transcript at Script Line.

As I understand it, Matthew both implies that there aren’t a lot of ways for Thomas to get himself sent home, while also confirming that wounded men are being sent back to England. So Thomas puts those two things together and sees getting himself wounded as his only option for getting sent home.

There’s an earlier scene that serves as a prelude to the above scene: O’Brien mentions having received a letter from Thomas and says “I don't think he’d mind coming home” to which Cora replies “Oh, I wish he could, O’Brien.” The scene introduces both Thomas’s desire to come home as well as his inability to do so.

I didn’t review the entire episode, but I don’t think there was any suggestion that Thomas was, as the PBS page puts it, “using his scheming ability to devise a plan to escape” prior to his conversation with Matthew. However, in the previous episode (the last one of season 1), he did “scheme” in an attempt to avoid being sent to the front altogether, by getting Dr. Clarkson to put him up as a volunteer for hospital work. That this didn’t have the outcome he had hoped is alluded to in a scene where he’s in the trenches, carrying a man on a stretcher together with another soldier. Before being shot in the head, the other soldier says: “I thought, ‘Medical Corps. Not much danger there.’ How wrong can one man be?” Thomas was acting upon the same thought in the previous episode.

So summarizing, I would say that Thomas had a plan to avoid being sent to the battle front, which failed. Once at the front, he didn’t have a plan to get away from it before his conversation with Matthew.