Curious if the 1.054 expected OG included the DME addition. It's been a while since I brewed extract, but the OG difference seems like a lot, particularly since your fermentable sugar content is part of the recipe and not subject to mashing problems. The only other variables I presume would be water volume and boil time. Maybe an issue with your hydrometer?
I will say that I've also struggled with English-style yeast, both liquid and dry. I've tended to avoid it in favor of generic American Ale yeasts (US-05 or liquid equivalent for example).