Frozen food is healthy if it is frozen in its original form - raw meat, vegetables and fruit that have nothing added. Same with tinned food if it is canned in its own juice or water.
Dried fruit is also natural but be aware that the sugars are more concentrated so not as healthy.
I avoid everything with sulphur dioxide as it triggers an asthma attack.
Lots of healthy foods have been processed - yoghurt, milk and cheese, as examples. On the whole, avoid the obviously processed foods such as sausage, bacon, ready meals and puddings, pies, pizza, sauces (such as curry and bolognese in a jar), cake, biscuits, crisps, sweets.
I'm sorry to say the list is long :(