If you order your main course in advance for FC or business class, as some international carriers encourage, you can typically also get an extra main course from the regular menu )after others have choses) to have a fish course and a meat course as part of your main meal. Something like this might also work for a cold plate and a regular hot meal, or you could ask for a leftover cold plate to be served later in the flight.

I have been able to snag a second meal around 5 times in my lifetime. Just have to ask nicely after meal services is complete.

You can certainly ask, no rule against it. As mentioned above, I would ask if there are any left over. Worst they can say is no.

You can bring food that is not liquid/gel/cream through security with you. Fruits, vegetables, sandwiches, hot dogs, trail mix, energy/granola bars, etc are all ok. You can also buy food and drinks air-side and bring on board.

If you absolutely will get hungry and sick if you don't eat extra, then you should bring stuff onboard with you. Yes you can ask politely and if they have extra they usually will help you, but economy class is usually catered to the exact load, with maybe one or two extra.

A subway sandwich with the condiments on the side, including things like tomatoes and cheese, will last quite awhile.

I've been known to bring small baggies/small plastic containers of carrot sticks, celery, cucumbers, fruit (certain fruits freeze well, like grapes, peaches, plums, and will stay cold for quite a long time - if you pack other stuff with the fruit, instant airline friendly cold pack), nuts etc for snacks.

Some airlines, usually in Asia, will provide cups of noodles in economy class. Since you are flying through LHR, most likely this wouldn't happen but you could bring your own (or anything else that could be heated up by adding hot water, such as oatmeal cups, easy mac, etc) as that is a request the flight attendants should be able to readily accommodate.

You can bring sports supplements such as protein bars, fruit and nut mixtures, maybe even a drink supplement that can be mixed in a bottle with water, etc.

It is possible to bring on sandwiches or deli food in general for consumption early in a flight. Note some selections may be annoying to fellow passengers.

The issue with carrying food that involves liquids, gels, etc. is getting through security rather than getting on the plane. There can also be a problem getting off the plane with meat, vegetables, etc. due to customs controls so eat everything that might not pass import regulations and leave the scraps behind.

Thanks guys for all the great suggestions. I think I will wait till everyone has finished eating and go at the back and ask them if I can get a second meal. The worst that can happen is they will say no and I will have to take the walk of shame but as long as they don't rub it in or see me indifferently then its all good.

Im not a big fan of protein bars or granolas but I will try getting something from subway with some less sauces.

You can generally bring any consumables on board, even alcohol. You just can't drink alcohol on board that wasn't served to you by a flight attendant.

As for what you should bring...depends on what you'd like to eat. Things that require refrigeration will stay good for at least a few hours, and I personally wouldn't have an issue with eating a meat sandwich that had been out up to four or so hours, and maybe up to six. Smell it and see - if it smells funny or appears/feels slimy, it's gone bad. So, you might eat the things that should be refrigerated first, if you choose to bring them.

Since you won't be able to warm anything up unless you find a very nice and underworked flight attendant, it's best to bring things that don't require refrigeration and you like eating at room temp. Nuts, dried fruit, fresh fruit (apples/banana/orange), perhaps a peanut butter and jelly sandwich (though PBJ might be problematic getting through security). Harder cheese doesn't really require refrigeration in these time frames, so cheddar is fine but I'd steer clear of soft/runny cheeses. Bread is good too, of course. Or just bring some powdered protein shakes and add water/milk from the FA.

You could bring some cold cooked noodles or a couple of Cup O Noodles - you can make them with some hot water, which the FAs will be happy to provide, but they tend to have a lot of salt which can exacerbate your headaches if you are getting them from dehydration.

I asked for a second meal when I was flying LHR-JFK on VS in Februari and got it. Just asked it to the FA after everyone was catered, and she let me even choose between the meals.
Although that flight was almost empty, around 80 passengers in the whole Y cabin of the A340 (I had a middle row for myself).

You can ask (not a faux pas), but it will depend upon how many meals there are (most catering is one per passenger). There are several posts on FT about "stinky" food (which is always in the eye of the beholder) being brought onboard. I would stick with blander, less aromatic foods. I am reminded, years ago, of a Delta Owly Bird from ATL to LAX on an early 747; eleven passengers and fourteen FAs. We were told to eat as many sandwiches (the only choice) as we wanted since they would be thrown away when we landed.

+1

Absolutely in economy. Never asked in C/F; always full from the meals or so horrified by them I didn't want to go again.

I thought it was common knowledge that McD's and Twinkies are not biodegradeable. :p

Seriously though, that is not the kind of food to eat if the OP is working out and presumably striving to stay healthy. Eat a big pre-flight meal and bring subs, sandwiches, trail mix, fruit. For economy class you just need to suck it up and eat your food cold.

The OP is lucky he's stopping over in LHR. It's one of the few places where it's possible to buy something relatively healthy ie not sweetened, or saturated in fat, or heavily processed.

They usually don't have extra catered. Most airlines have food for purchase. You'll get better food by buying some before you go on board.

I've asked in the past while seated in Y and gotten an extra meal. I always make sure everyone has been served before asking though. It certainly can't hurt to try, but I'd bring extra snacks just in case.