New idea, unsure of how to start

[FatandSassy]

So DP and I have an idea that seems pretty good. In order to make it happen we would need to find someone to manufacture the custom items we would want to sell. We would have a brand and (hopefully) the potential for products to be sold in retail, eventually.
I've been reading up and spoken to a few people and they say start as a sole trader and register and go from there: our problem is we have no finance and in order to (ideally) get some we have to produce a business plan and do research etc
Where do we start with this? We currently have no idea on the costs involved, since they would be custom items do we order and pay for them and go from there or do we just do it all "hypothetically"? We would need a website and a business account etc - presumably these are costs we can work out without actually doing them. And the same for the brand logo etc. We will need an address for delivery and postage. Do we just get a P.O. Box for that?
Sorry this is so long, it's only a few of the questions we have but it seems such a big challenge!

You need to find out how much the product will cost to make, how much you can sell it for, research whether there is a market for it, get quotes for things like insurance, you cannot have a PO Box if you are retailing online, read up on the Consumer Contract Regulations, if you are planning on working from home you'll need to speak to your home insurer and mortgage provider or landlord. With regard to developing the product, you need a prototype making and you need to see if you are manufacturing in the UK or over seas. You will probably have to visit the manufacturer in person and expect to need to place a very very large order. Don't forget to factor in customs fees, VAT etc, and speak to an accountant.