You don't need a referral. Just call a local physio and ask if they have ultrasound, then go and tell them you need it on your boob (then watch their expression lol). They might have to look up intensity levels but mine found it easily on the internet which confirmed my GP recommendation.

My blocked duct was from when I had unclipped my bra and the fabric was still under the breast. I had to literally take my whole bra off to feed for a while until i cleared the blockage.