right, lets have a think of tips i can pass on, firstly, there are a few specifically craft based forums you could join, i am editor of the business board on 'crafteroo'. there are lots of others there who make stuff to sell, patterns etc. you will be able to get more specific advice from there about suppliers, and also online venues for selling, like etsy, folksy, dawanda, big cartel etc.
it's a shame to say this, but the cheapest place to get things like packaging, paper (for if you are going to print your own leaflets, patterns etc to go in the kits), thread, will be the far east, although, you may need to buy in bulk.
there is a place that is uk based called craft creations who i have bought packaging from before, they are pretty cheap.
for business cards and other bits of stationary, i use moo, their mini cards are very reasonable, £9.99 for 100.
for fabric, have you thought about going to the festival of quilts at the nec in august? i went last year, it's mainly fabric suppliers. it's an excellent opportunity to see lots of fabrics, all in one place,and be able to talk direct to the shops who have stalls there about deals on larger quantities.
one thing you can do, is think about sending a press pack to all the uk sewing and craft magazines, send them a free kit, they might review it in their magazine, free advertising if they do!
hope that helps!