You should take this to the business and marketing forum. They can help you consider all your costs, computer, overhead...
You might not want to bill it hourly, but you need to figure out how many hours you are going to invest in this, designing the characters, animation, revisions, narration, editing the narration. Just my opinion, but 600-800 seems way low(that's about 900-1200 US$). If someone designed the characters for me(in layers preferably), 16 hours would be a pretty low estimate to get something like that pulled off, but I think that's more of 24-30 hour project for 1:30 in animation.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.
If your work is of a comparable quality, £600-800 seems like a really, really low estimate. Personally I would quote more than that just for the initial character animation.
It's kind of beside the point, but once you've paid your taxes, even with 4-5 projects like this per month you'd still be earning below the national average wage. For a skilled profession, that seems ridiculous.
I made this mistake so many times when I started out - don't be afraid to aim high in terms of quotes. Be prepared to defend your work and your right to be paid fairly. Remember they'd pay many thousands more by going to a prod house. This guy has some useful ideas: