Lol 5 hour walk. Child. Get your mum to pack you a sand which and a water bottle.

A 5 hour hike? That's nothing. Just take plenty of water, maybe a powerbar or some jerky, and make sure your phone is fully charged.

Haven't been to Snowdon, but I have done Ingelboroug. And a few other walks in Yorkshire. Water is more important than food. Makes sure you have more than enough of that. Eat something with a lot of carbs the day or morning before. Like pasta. Slow release of energy. You might want to take some other high-energy foods to snack on while you're there. Like chocolate, in case you start feeling tired. And of course take a regular meal with you to have maybe half way or before you descend. Not food related but try and take hourly breaks. No more than 10 minuets.

Pro-tip: Don't fall.

When I hike we usually go about a mile every 20 minutes. Bring snacks, and lots of water. Nothing will ruin a hike like being thirsty the whole time. Of course it's also dangerous. I usually take a bag or two of m&ms and some trailmix/nuts. I don't know what the hikes are like over there, but here hiking boots are pretty much a necessity. You're feet would just die without them. Also water.

Eat my shorts

When you cum upon a bear...don't

So you're not going innawoods and camping there? Being beef -blam!-ing jerky, you're a growing boy. Eat it or add it to soups. Also Plain flour can be mixed with water to make dough. Though because it's only 2 days long would be Jerky, Protein Bars, maybe a MRE (Not British MRE's, that shit be disgusting. I mean 100% American born MRE) Bring Iodine drops as a antiseptic and water purifier, shit load of quaze. Rubbing alcohol or strong Vodka to clean wounds Also bring a fuc­king Leatherman, boy. If I see you with a Gerber or Schrade multitools I will stab you in the mouth. Swiss army's are classy, but don't have pliers

I'm sure you already know this, but always bring more than you think you'll need. Bring the essentials that would come in handy in case your 5 hour trip turned into an accidental over-nighter. As for nutrition, your best bet for your time frame would be energy bars and the like. Clif Bars are great, lots of different flavours, and they actually hit the spot unlike some others out there. I used to eat them all the time at work if I needed a snack before/after lunch break.