What I am about to say below has little bearing on whether you can build or find a way to do what you want to do in a cheaper manner, but it is something to keep in mind, since you are in school. This is information that you might just "blow off", but I promise you, if you keep it in mind, you'll be way ahead of the game...
I currently work in Maplin and earn around 300 pounds a month (do a bit of basic math) and it would take me about 5 months.
There is nothing wrong with deferring your wants and saving money. Have some patience, take the time to build savings, and you'll be that much further ahead in the long run.
Any better, I could do with a contract deal and say pay monthly 200£ or something.
So - you're saying you'd rather go with unsecured credit to purchase such a machine? May or may not be a bad idea; if there's no interest, and you can afford it (will 100 pounds left over be enough to get you by each month?), then it would be a good deal. However, if there is interest - you'll end up paying more than if you just saved and waited. How much more would depend on the level of the interest, and how much you paid each month.
Unsecured credit can be an evil thing. It is OK, as long as you are able to pay it off completely each and every month. It can become evil very quickly, though, if you only pay off the minimum needed each month.
My advice: Make a budget. Live within that budget of expenses vs income. Set aside a portion of your budget to go into savings (rainy day fund - because there -will- be rainy days). As you get older and have a career (after you graduate), you'll want to build your savings so that you have at least 6 months worth (if not more) that will pay you what you earned monthly if you lose your job (this money is also called an "f-u" fund, because it will allow you to tell your old boss to stick it, so you can easily move on without any worries).
If you are able to have such a savings, and don't have any unsecured debit (ie, credit cards or other similar credit instruments - mortgages are ok, though) - you can live very comfortably, without much worry. Believe me - my wife and I had a pile of debt (not as large as some people, though), and when we paid it off, and had savings in the bank - the feeling is wonderful. I recently went through a period of a month where I didn't have a job, and what passes for unemployment insurance here in the states didn't help much (to be honest, its been relatively worthless); knowing I had savings, though, made it bearable. Knowing I didn't have a credit card payment, or any car payments - that I just needed to keep the roof over our heads paid, and water/electricity on - that was bearable. My wife was still working while I found new employment (I just completed my first week at my new employer) - so we still ate ok (not that it really matters; we probably have 3 months worth of food in the house and freezers).
Take this to heart now, while you're still young and have a bit of a "head start" - you'll get your 3D printer in time - but you'll also get a lot more if you follow the above advice and start with it early.