I’ve never had a British Seagull but have had several
British bikes and I’m familiar with the control.
The inner part of the cable may have broken at one end,
was reattached, and now its too short to get the full
range of movement. If that is the case shortening the
outer part of the cable some may solve your problem.
Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.