Airport Arrival Time

The standard airline requirement is 3 hrs. You might cut it to 2.5 but that would be about it.

Check what your airline recommends. London is not Schengen (so you need to allow more time than for a flight to a Schengen destination), but it's also not a trans-Atlantic flight (so 3 hours is probably overkill).

Baggage Check in procedures must be completed 45 minutes prior to departure (British Airways, Alitalia) or 40 min (Easyjet). You must be at the gate for boarding 30 minutes prior to departure (all carriers).
If you are at the check in counter in line 75-90 min prior to departure for a flight to London it will be more than sufficient. If you fly Easyjet, I believe the check in is done only online now, therefore at the airport you just drop your bags at the counter. I've never showed up at Malpensa 3 hours prior, not even for transatlantic flights. Two hours are sufficient even for those flights. As a matter of fact for European flights I believe the check in opens 2 hours prior to departure.

Three hours seemed too long to me, too, so I checked with BA and learned that it is 20 minutes or 40 minutes, depending on the airport. Thanks!!