If you don't want to sit in a queue for too long, I suspect your goal is to shorten travel time ? Because Flying or taking a coach are so not comparable on this route (and maybe on any route).

• The coach is not confortable (worst if sitting next to someone unpleasant for many hours) and I would recommend only if you're on a budget (even if considering the distance, the cost might be high too) or want some adventure. I had situations where I had to wait more than six hours in Dover for the next ferry. Immigration will not be your only problem (delays, weather, etc).
• Eurostar will only take you as far as Brussels or Paris, from where you'll still need to train, bus or flight, adding travel time and separated bookings (risky.) If you choose that and take a train from Paris to Italy, ALWAYS choose the Frecciarossa over TGV.
• Flying is by far the most logical and best option in terms of travel time, confort and cost. Italy is one of the larget markets from the UK, it will not be hard to find a cheap flight from either STN, LGW, LTN, LHR or even LCY.

As for your fear of hours of queuing through immigration/security. So far this year, I did not wait more than 30 minutes at LHR (Terminal 4), and not more than 2 minutes at LCY. Many friends had no problem either.

Without status or flying private, there is no magic. Delays, complications and queuing are part of travel, especially in summer, since the dawn of time. I believe, someone called Homer wrote a book about a guy called Odysseus, who travelled from Troy to Ithaca back a few years ago. It took him 10 years ! And you wouldn't believe what happened to him, if you compare, I believe queuing a little does not hurt. (He might have been Miles+Bonus gold, though.)