Ok... Where in the UK are you?
Because of your location on the map, you might, and probably do have hard water. In hard water, the BS will not be able to break down completely. This means that you can't rinse it out completely. When a base like Baking soda is stuck in your hair, it'll just keep reacting with whatever moisture is on your head. No matter how much ACV you use, or how much your rinse, the BS will not get washed off, so it'll just keep making your scalp more basic that it should be. This causes itching and brittleness.
There are ways to stop this. You could A) boil you water and very gently pour off the pure water and leave the minerals behind. Don't pour fast, or all of the water off, because that'll just remix the minerals. Or B) make or buydistilledwater. I'm not sure where you can buy it there, but making it isn't that hard. Or C) get a soft water purifying shower head. This is the easiest.
With the pure water that you either bought,made or boiled, though, you shouldn't have anymore issues with BS not fully dissolving