More than the yeast strain, I hear that mash temp is critical to set up the yeast for success - low and long. But for an English style BW, you can finish it with some Champagne yeast, if the yeast selected happen to poop out. The 12.5% ABV is pretty high for most standard English yeast strains, but they may be up to the task.
I wouldn't use champagne yeast to finish. I've done that in the past and found it to not be as neutral a yeast as people like to think.
Make a big pitch of whichever yeast you choose, oxygenate, and that should be enough.
I like wy1968 and have found it to attenuate well in a big beer. It's what I use in my old ale.
I'm also not certain that 2.5 lbs of crystal is too much for ten gallons. But I also use sugar in big beers to get them to be drier and more drinkable. So take that for what it's worth. Maybe cut back on the crystal if you're not going to use sugar.
I cut the crystal down to 2#, but I will wait to see how the yeast attenuates before I add any sugar. If I need it lower, I'll add some invert sugar after I get a stable FG. I don't think I'll have any issues getting the beer to where I need it.
Tap: Brandy Barrel Sour, Brett IPA, Tart Cherry Berliner, Sour Coffee Porter, Witbier, Helles Bock, Helles Bock Braggot, Strawberry "Nebraskambic," Raspberry Flanders
Secondary: OatRye Blonde, Plout Sour, Blueberry Ginger Sour
Primary:Belgian Dark Strong, Scottish Wee Heavy, American Session Wild Ale, Brandy Barrel "Lincolnambic"