I think that if he's going to be useful it's going to be in counter-attacking innings to give failing innings respectability and crucial breakthroughs with the ball, rather than as more of a support player forming partnerships and building pressure. Even Good Pandya, if it all works out for him, isn't the player you're hoping for there IMO.
If Pandya does this plus what I described, then that would be perfect. If we're on top, he just needs to provide support to ensure we don't lose any advantage. When we are struggling, he puts in a performance that helps arrest the slide.
However, if he only performs the crisis man role and not the supporter role, then that's kinda useless IMO. Because I don't think he's capable enough with bat or ball to take us from a losing position to one of parity. He'll just score some consolation runs, or take a wicket or two. It will still require other players in the team to step up to turn the game around, and I just don't believe our team has the players who can do that overseas. Kohli and Rahane with the bat maybe, and Yadav/Shami if they hit rhythm in one of their spells. But we haven't been that good at coming back from being behind in overseas test matches.
Similarly, if we do happen to get a dominant position, it would absolutely suck if Pandya can't press that advantage forward, and instead ends up throwing his wicket away or bowling a bad spell. I think England are far more likely to win the opening exchanges of these matches, and so it won't be often that we'll be ahead of the game - if Pandya screws up the few times that we are, it might cost us the few chances we have to actually win.
He won't play a role in setting us up for victory, but he will play a crucial role when it comes to either maintaining our advantage or coming back from behind. If I had to choose for Pandya to excel in one of those two, I'd choose the former. I can see how he'll end up with better numbers and a more favorable impression with the fans if he does the latter tho, since I expect India to be behind the game for most of the series.