Siobhan, who began her career at Keynsham Swimming Club, where she is still a member, clocked 2.07.57 sec to qualify fastest by more than half a second.
In doing so the 20-year-old, who trains at the University of Bath, broke the British record by 7/10th of a second.
The final will be staged tonight (early hours UK time).
Keynsham Swimming Club said on their Facebook page today: “Brilliant semi swim for Siobhan. She broke her British record and goes into the final as the fastest. Go our girl.”
Meanwhile Longwell Green’s Claudia Fragapane, 18, is part of the British women’s squad who have qualified for the women’s gymnastics team final which takes place tonight.
She is a member of the Bristol Hawks Gymnastics Club.