Eversorubbish with the hints and tips.

So here's one.  Very easy.  I can't see why the same principle wouldn't work with most flags (Andorra might be tricky).
It's a cake, just in case it's not obvious.  For the cake itself, I used one of those disposable foil roasting tins to bake it in, as I don't possess a giant sized rectangular baking tin, for some reason.  This worked brilliantly and I may adopt this procedure for all baking from now on, thrilled as I was to find I didn't have to wash it up afterwards.  I made 3 times the normal cake recipe, threw it in the oven for 6 days (it seemed like 6 days) et voila!

Then I bought about 400 family-sized packs of M&Ms so I could get enough blue ones to cover the cake -  you could use any brightly coloured sweets but I really wanted that vibrant e-number blue for this one - along with some utterly ghastly strawberry sour ribbons and white mini marshmallows.  I covered the entire cake with plain white icing, gave it a few minutes to dry off, then started with the red cross and built it up from there.
It's suitable for any WWII obsessed 12 year old you may happen to know, and mine was very pleased with it. But I was just thinking how cool it would be to knock one or two of these out for any street parties you may be planning for June (Queen Elizabeth's Diamond Jubilee, for anyone outside the UK).  You could even leave it in the baking tray for easy transportation.  I'm on fire this morning!

And here he is, bless his little camo socks...