Let's pick one example of a component that doesn't feature in the inspection* , but is worth servicing at the recommended intervals. The timing belt .
This is the belt which drives the camshaft, which in turn opens the valves on the engine's cylinders at the right time. If this component fails, the pistons will collide with the valves and the entire engine will be wrecked, so it is vitally important that it is changed at the manufacturer's recomended intervals. When buying a used car, one should always check when this component was changed.
The ONLY test for the engine on the British MOT inspection is emissions. A worn timing belt will not affect these in any way, right up until it fails.
Not changing / topping up the oil probably wouldn't affect the emissions, but it might. Particles in the oil could increase the wear on the piston rings, and eventually the engine will then start burning oil that got past the piston rings (blue smoke in the exhaust.) Failing to change spark plugs probably would affect emissions quicker though (as well as reducing the lifetime of the engine.)
The remaining tests on the MOT inspection are all to do with safety. The government only cares that your car is safe and has reasonably low emissions. AFAIK there are no tests on the gearbox at all, and I've always thought it would be interesting to present a car without a gearbox for the test.
For items not in the inspection, deciding to get them serviced or not is entirely up to you. If the car is new, you will want to stick to the recommended service intervals to avoid invalidating the warranty. If it's just out of warranty, you'll probably still want to change the oil at the recommended intervals to conserve engine life. The garage will probably continue to suggest that you keep up the manufacturer's servicing intervals. Although this may be more in their interest than yours in some cases, I don't think it qualifies as a "scam."
On the other hand, if the car is 15 years old, its value may be so low that it's only worth servicing the things required by law. It's still worth checking yourself that all the oils and fluids are topped up though.
*the inspection is still known colloquially as the "MOT" in the UK, though it is no longer administered by the MOT but rather by the VOSA.