I seem to remember one of their unique products was an anti howl round device for live applications that shifted the pitch of a signal by a few Hertz. I believe it used SSB (single side band) techniques to achieve this.
I had dealings with Surrey Electronics, as I was an employee of the company that distributed them in France.
The only product we had success with was the Spectrum Shifter. It used phase-shift circuits to provide quadrature outputs of the input signal and analog multipliers to provide the two side-bands (upper and lower) in combination with the LFO.
The proposed usage as a feedback eliminator was instantly rejected by musicians because they heard the disastrous change in pitch.
It did not succeed in address either, because it gave the speaker's voice a warbling quality that made them appear slightly sick, or tipsy.
It was used in music production, where it created some kind of phasing effect. Unfortunately, the stereo outputs contained the same effect with opposite polarity, which made it disappear when collapsed in mono.