I see no reason they shouldn't be made to the same tolerances and standards as pieces for any other bow. If I grind a single thick piece of osage for a basic backed bow, it's easily tapered and thicknessed to the same tolerances that I grind lams for glass bows. In fact, doing so ensures a predictable, quality glued up profile, reduces the chances of limb twist, and gives a good jump start on tillering. English longbows, because they're narrow and deep, are especially susceptible to limb twist and misalignment and should be made as accurately as possible.
I also see no reason that anyone who's set up to grind good quality lams for other styles, couldn't grind them for an ELB. What's the big deal? They're thicker? Longer? Different tapers? So what? They may not know the exact lam specs because they're unfamiliar, but if you tell them what you need, they should be able to accommodate you easily enough. Have you contacted anyone about it?