Well,first of all,Cooper's cans are intended to make 23L,or 6.072 USG. About 5 imperial gallons. Cutting the total volume to 20L,or 5.28USG will up the OG & ABV% by a few points & improve flavor a bit.
But you have to keep in mind that this is an English IPA. It's mostly bittering,whereas it's American counterpart is mainly hop flavor & aroma. I'd ditch the dextrose (corn sugar,not just powdered sugar) with an equal amount of plain light DME.