Low flow with shower head on

There will be a flow restrictor/ filter in the hose or shower head causing the low pressure! Can usually be removed with long nose pliers

Is the washer obstructing the inlet and is there a potential blockage?

The most common cause of low shower pressure is a blocked showerhead, as a result of limescale build-up. These blockages obstruct the water flow, leading to reduced pressure over time. Most showers are fitted with a filter, protecting the pipework from debris, but this can become partially blocked by scale.

It sounds like it’s a flow restriction problem see if you can purchase a low flow head that should help

This can be due to the heating element scaling up. The scale breaks away and runs through the hose to the head. The holes are too small to allow the scale to exit.