I'm planning to study the same course! I had a chat with their Admissions team, who were surprisingly knowledgeable about the course, and slightly guarded about who they let on the course - which I took to be a good thing. I have a similar background to you - BTEC A&D, then went in another direction; I'm also very keen to get into something creative, having been away for so long. I want to study for personal development, not the piece of paper at the end of the course (although that will be helpful in being taken seriously!), and as I'm unable to (financially) go into full time education, IDI makes sense. Of course, there are no part time GD courses in London at the Foundation level up, so IDI are also my only option. FYI - if needed, you will be able to take a student loan to cover the course fees from October, but not before then (changes in uni fees have changed the student loan structuring from Oct only).
I have major concerns about them though - I can't find any meaningful reviews from students (outside of the IDI website), and am still very unsure about distance learning with a subject like this - I'm incredibly worried about how this will restrict the teaching. I'd love to hear the views of any of the more senior members of this forum on this.
So, to answer your question - can't give you any meaningful advice, but I think I'm going to take a punt.