If you eat properly, particularly focussing on low-medium GI meals and slow-release carbs (porridge for example) there'll be no need for fuelling for runs of that length - the body will store enough glycogen to cover that.
People do sometimes use energy gels in races but I suspect that is more due to the effect that if we taste something sweet - even if we spit it out - perceived effort is reduced.
Training programs develop a number of different things.
One of these is heart and lung fitness, and for someone with a fairly active background, this may be quite well developed, so the early C25K sessions don't feel tiring from that point of view.
Another is adaptation to running for the body's support structure - bones, ligaments, tendons, muscles etc. This usually develops slower than cardio-vascular fitness and someone with a non-running background may well not have any head start at all on these adaptations. (The flip side is that once done they tend to hang around, so people coming *back* to running are at less risk of injury).
Hope this sheds a bit of light on why the C25K program is the way it is.
It doesn't say you have to sit on your bum in between. Just not run
If you have a look around you'll find out there are loads of marathons - but if you still end up having your heart set on London, then join your local running club. All clubs affiliated to UKA get a number of allocated places for London and competition for "club" places is usually less than for ballot places. In other words, it will be much easier to get a place for London if you join an affiliated running club
I say "job done" - I'm on the bench at the moment so running 0 kms a week, which gives me much more time to sack off the gym and cook and eat Victoria Sponge instead... funnily enough my clothes are a bit tight.