The GWR timetables only show trains from Victoria to Kensington and Southall, which were probably standard gauge. However Alan Jackson's book, London's Termini, also states there were occasional services out as far as Windsor as well as a Sundays only service to Henley. I'll leave it to others to guess whether those might have involved broad gauge trains.
However Jackson also mentions slip coaches from GWR expresses from Birmingham and Bristol being worked to Victoria in the early years. Surely the Bristol one would have been broad gauge.