Well it's a question of your view, my attitude is it sounds like you're paying with money you don't yet have and assume you will earn, however reasonable that assumption, in my view it's bad practice.
Finance has a place when it allows you to spread the cost for cash flow purposes or delay the capital expenditure but £40 a month for 3 years doesn't seem wise on either grounds. If you can't afford that now then, in the nicest possible way, you're buying a luxury product with money you don't have, which is questionable. I don't want to lecture but £40/month for 3 years can't be logical unless you're really strapped for cash (in which case, bad idea, full stop), surely over 12 months would be more logical, not to mention cheaper?
And as with all these schemes, it preys on the stupid and lazy somewhat, a bank loan for the same amount will almost always be cheaper...