Your best place to start is to contact a Professional Registration Advisor who can help you with this.
In terms of supporters, this page outlines who your ideal sponsors will be.
Without the support of a registered engineer, it would be difficult for a panel to assess an application with a sufficient level of confidence. That said, your advisor will explore options with you.
You could also consider asking for an IET mentor who could spend more time working with you on a development plan towards registration.
CEng MEng MIET