Don't count on being able to heat anything up. A cold Subway sandwich will last for hours as long as you don't get one of the soggy over-sauced ones (e.g. Sweet Onion Chicken Teriyaki, etc.).

If you are laying over at LHR there are plenty of places to buy cold sandwiches, baguettes, etc. and of course you can take them aboard with you.

If I were you I would eat as large a meal as possible at the gate before boarding, and pack plenty of non-perishable, filling snacks with you. This may limit you from bringing "meal" foods like meats (I avoid those as they would stink up the plane anyway), but bring protein packed foods like nuts and energy bars. Hearty, non crumbly breads like bagels would transport well, too.

If you fly CX from LAX to HKG, you can ask for cup of noodles in between meals as many times as you want. :p

On the other hand, when we asked for some chips (in between meals) for my young son flying business class international, 2 flight attendants brought out many bags of chips, opened his tray table and lay out a table cloth on top. :p

Big eater - can I request second meal?
 

If you lived in Germany you could order the extra meal to your house and bring it on the plane...

http://www.nbcnews.com/business/travel/germanys-lufthansa-offers-flight-meals-home-delivery-n208746

Wasn't there just an article about being able to buy to-go picnics in London that you can take onboard?

Found it!
On-Board Picnic Available at Heathrow

You can design your own menu to supplement the airline meal, and probably have some other passengers looking at you enviously.

I agree with everyone who suggested you pack food to take along, whether sandwiches, fruit, or high-energy snacks. If you make sandwiches at home before you go, put a lettuce leaf on the bread before you add the filling; this will help keep the bread from getting soggy.

I once was sitting in the second to last row in economy class. They ran out of food. However, they did raid first or business class and split up those meals among several economy class passengers. I got a bowl of soup, bread, and some nuts. Someone else got a main dish only.

On a few occasions, I've been unusually hungry and asked for, and received, a second meal in economy. I asked only after everyone had been served, and it was clear from the way I asked that I thought it would be a huge favor and not an entitlement in any way. The response has never indicated that it was a problem.

Of course, there's no guarantee that there will be extra food, so if you want to be sure of having extra food you should bring it aboard yourself.

you can always try asking for complementary nuts or on some asian flights cup noodles.

If you bring food on international flights, be aware that incoming customs rules probably require that you dispose of any uneaten food.

Try calling the airline and explain that you need an extra meal for "health reasons." Maybe they can provide one for an extra charge.

I once saw a guy in J who had ordered a kosher meal. Evidently the airline had loaded both a vegetarian and a chicken meal and when the FA asked which one he wanted he took both - and they weren't small meals.

This topic brought back some of my old memories about this :D
Many years ago I flew SFO-HKG round trip by CX, I asked the flight attendance for an extra meal... both worked. I looked very young back then, maybe that helps. First time succeed :p
I'm a big eater too. When I fly long-haul international flight in coach, I occasionally ask for another meal. Sometimes I could be lucky, maybe some other passengers skip the meal. Just a matter of probability. Most recent experience was flying OSL-IST operated by TK. That was the best inflight meal I've ever had in my life. I asked for another meal. No dice :(
When flying biz/first class, there always seems to be infinitely amount of food...
Just ask, don't feel bad about this - nobody will remember you.

I do not ask even in FC, but a FA could maybe do a carry out run, or call for delivery...

I would just carry a few donuts...

Please allow me to direct you to my blog, where I've eaten burgers which are days old. I've yet to die!

I ask my FA and she gave me more foods. I was so extremely hungry. I ate pasta and it was so delicious! I had no problem for me at all.

I flew out DTW-CDG. She gave me a dinner and I ate tortellini and I am really like it lots. It was so delicious! I call FA button and I ask her if she does have more tortellini and she went to back the gallery. She look for it and she came back right at me.

I imagine easier in Biz or First. Was it economy? Amazing if it was. Just wondering....