Hi Lizzie
Surgeon first, and my surgeon referred me to my oncologist. My GP (first port of call) decided wisely she needed someone who knew more about breasts - her second try could see me in two days. My surgeon arranged the mammogram and ultrasound, then a biopsy (all in one session). The result was a mastectomy and axillary clearance (removal of several lymph nodes). In some cases, adjuvant chemotherapy (to reduce the tumour size) is proposed before surgery, but mine was quite advanced, so surgery first, chemo for the mop up.
My surgeon and oncologist worked quite closely together, with frequent discussions about patients which I found very useful and reassuring. It's not essential though. Best wishes.