I think this book is the best aid to start your own business in interior design. Lot of ideas and exact form or strategy help you in this book. Some useful title: Design consultant or product driven designer are u?, Working with builders, Basics equipment to start, Samples, Business plan, Money sources, Forms, Contract, Advert, What to charge? Appointment-meeting step by step.
The book is fully America specific, but I think really easy to adopt to the UK market.
The main line all over the book an curtain and window blind maker/designer "company". And a bit too much explanation about this and less than the designer "life".
But I really recommend it really organized and full of help to the start.