There are no low-audience mainstream channels. TV ads work in bundles and are sold via media agencies. You may even see local ITV affiliates advertising the idea of TV advertising. But selling small bundles costs more in admin than it brings in revenue.
There are local 'windows' for local shops and other businesses, but a single ad or even a series of ads are very unlikely to sell much. What they really want is to place a big contract for months or even years with a media agency that guarantees a steady income.
There are three media sales outlets - Channel 4 (who also do ad sales for smaller outlets) ITV, and Sky. With the move to online viewing, TV buying is increasingly algorithm-driven, especially in Connected TV (CTV): i.e., streaming services like Hulu, Roku, and Pluto TV sell dynamically inserted ads based on user profiles. And so-called Addressable TV and Sky in particular, in the UK, lets agencies target different households during the same programme.
So today, broadcasters are becoming data-driven ad platforms, and agencies are morphing into tech intermediaries rather than just bulk buyers.
All that means the SME should launch its own YT channel and produce its own programmes discussing its market or technology issues.
Broadcasting in the UK is beginning to have to think the unthinkable - the future is not channels or stations anymore! (But that, as they say, is a whole new and very different can of worms!)