Helen Hughes's reply:
The route in will depend on which area of interior design you are most interested in pursuing. There is the more interior decorator side of residential interiors or the more architecurally based field of commercial design. As a mainly commercail designer I would highly recommend a BA at one of the leading unitversties such as Nottingham, London Met, Westminster or Kingston-U-Thames. Nottingham is 4 years but that encompasses a 1 year placement in industry which I can't recommend highly enough. It will give you a great advantage when seeking a job after graduation. I'm sure my list is not exhaustive of great courses and I would suggest visting degree shows and open days to any universites that are of interest, speak with the tutuors and the students and get a feel for what fits best for you.
For any university place you will need a folio of work and relevant qualifications to apply. I made a career change myself in 2000 and decided the best option for me at the time was to study for a BA. To prepare myself for that I undertook a part-time course in 3D drawing to assit with the creation of a relevant and current portfolio of work that I was able to use to enhance my previous portfolio from an ND in general art and design.
There are also many other options open to you now with so many online folio creation sites and image driven sites such as instagram and pinterest where you can begin to define your personal interests and style which will enhance your application to any course.
If commercial design is not your goal, then maybe contact the BIID for further information on courses more geared towards residential interior decoration.
I hope you find this useful and wish you luck in your change of career.