Macmillan New Writing is a part of Pan Macmillan Publishers, a mainstream publisher. If you read the page I’ve linked to, you’ll discover that they now accept unsolicitored manuscripts. The contract will be different to what an agent would be able to negotiate for you, but I feel this is a way for an unpublished author to seriously get their work recognised.
If you are interested in submitting a manuscript to them, I strongly advise that you read the website carefully and find out more about the program beforehand.
They only accept adult fiction, so at present none of my manuscripts fit the guidelines…but that won’t always be the case.