hello-
reverse engineering armor is a good way to learn how to make stuff you need. it also gets you thinking about it differently.

2 mm is pretty thin but if you're gonna just be getting whacked by boffer weapons it should be ok. next time you might want to try some veg-tanned leather around 8-10 oz, then you can form it like the tutorial shows. if you use a leather jacket it is garment leather which is tanned to be all soft and supple, not ideal for armor.

most of that tutorial wont work at all for your project (again because it's garment leather) but you can use the pattern and spacing guidelines.
DO NOT try to dye it! it will just rub off on your skin.

might want to be careful with the leather lace, as some of it is prone to snapping when you try to tighten it (i found out the hard way when i tried to lace up a corset i made) sometimes it is just better to use plain black boot laces.

i work at a leather store here in massachusetts, (tandy leather factory) and we give free classes here, so i imagine the one in the UK does as well? unfortunately the tandy there is about 2 hours away from you, but if you can make the trip you'd probably learn alot from the trip.

i can help you out with some of the less serious stuff.. i've made a full set of armor for my boyfriend, and a scabbard, a small sheath, a corset and bustier for me, etc etc. i'm learning more the longer i work there and can point you in the right direction as far as what products are crap and what is actually worth the money

www.armourarchive.org has all that you ever wanted to know and more about armour. you can go ask questions and read discussions and learn quite a bit.