Use Extra Light DME with a 37 ppg / 3 L rating (the closer to actual 2-row, the better). Muntons is a good brand. Don't go overboard with a ton of converted specialty grains and high percentages of crystal.
If you're brewing something like a pale beer over 1.065 OG, then replace 10-15% of your extract with a flameout corn sugar addition. Seems high, but not when brewing an extract beer. All Grain recipes can contain 5-7% without issue, and Partial Mash about 7-12%. Use a high attenuating yeast like 1056/WLP001 or WLP090. Using fresh yeast, make an appopriate sized yeast starter, following all the right processes. Use some yeast nutrient in the starter and the boil.
Ferment in the mid 60s and then let free rise to about 68 F after 3-5 days. Keep in the primary for 27-30 days total. Some temp. fluctuation after that time is okay, but try not to let it drop/rise drastically from 55-75 F.
You should notice lower attenuation if you follow these steps, but I doubt you'll ever hit the mark of an extract converted all-grain clone.