My child was a picky eater from a young age and looking back I wish I hadn't listened to everyone's opinion. We got so much bad advice which made the situation, and behaviour, worse. 'Picky eating' can be indicative of a more serious condition called ARFID (Avoidant Restrictive Food Intake Disorder).
It is actually something that is related to their development, not whether or not you bring your own food from home!
Only much later did we realise that my child was on the autistic spectrum, but it wasn't apparent until they were in school (6/7 years old) when other issues became evident. They could do all the things that I thought autistic children couldn't. Autism is much wider and complex than anyone understood before and if you can look at your child's behaviour in a more holistic way it will help you to help them in the long run. For instance we would have got them a neurodevelopmental psychological evaluation much younger.
Unfortunately most people in the NHS are siloed and we wasted much time with the typical medical routes... behaviour is a complex one.
Look at ARFID Awareness UK. The person who is the expert in this area is Dr Gillian Harris. She explains 'picky eating' much better than Ella or any dietician.
http://www.foodrefusal.co.uk/who-we-are.html