That's really strange - generally if a breast clinic feel a patient requires a specific imaging test they would arrange it directly.
They would not usually bounce it back to the GP for a referral.
Rules can be different between health boards but GPs cannot refer directly for some tests, they would have to refer you to a hospital clinic, which would then arrange imaging as necessary.
In this case it would be the breast clinic!
Excuse my ignorance but by ASD do you mean autistic spectrum disorder? I can't see why this would have any bearing on your breast imaging.
Unless you mean they have recommended an MRI scan and you would find the noise and being in an enclosed space difficult to tolerate?
Even so, the breast clinic would normally arrange alternative imaging directly.
Is this routine screening, high risk screening or have you got any symptoms e.g. a lump?