M&S has a huge selection of ready meals, so it will be inefficient and time-consuming to analyse them all. Therefore, I am going to post a guide how to choose the healthiest option from the ready-meals counter.
First, I personally, would recommend to avoid ready meals unless absolutely necessary. Processed foods are made to be likeable by everyone and this is done by 'improving' the flavour, usually with the addition of salt, sugars etc. However, it is not a problem to get yourself a ready-to-eat mac&cheese, if Just don't make it a habit.
How to choose the healthiest option
First, think about your own diet and what you ate during the day. In the end, it's the balance that's the most important. Try to choose something, that will balance your diet during the day. For examples, see below
Fruit & Vegetables / 5-a-day
WHO recommends to consume at least 400 g a day to lower the risk of various health risk problems, like heart disease, stroke and some cancers. Therefore it can be roughly divided into 5 portions of F&V per day, that you should consume as a part of a healthy diet.
If you feel like you didn't have enough fruit and vegetables during the day - get something with vegetables, e.g. Vegetable Curry with Pineapple.
Traffic light labelling
I previously answered in detail how should you read this type labelling. Here is a description what each colour means again:
When choosing the healthiest, make sure that the food is labelled GREEN and AMBER, and avoid RED.
Fish
It is recommended that you should eat at least two portions (140 g/4.9oz each) of fish a week, including one portion of oily fish. Most of us don't have enough fish in our diet, so when you get ready meals - maybe this time it might worth it to get a ready prepared fish? For example, you can get Fish Provensal . Also, one of your 5-a-day and green labelling is everywhere.
Meat
Processed meat (e.g. sausages or ham) is associated with higher risk of cancer. Some meats (e.g. bacon) have high amount of saturated fat. Meat is also a good source of quality protein and some vitamins (especially B12). So what meat should we choose?
When you buy ready meals with meat, check the label on how much fat it has. It can be easily done with traffic light labelling - avoid getting meals that are labelled red for fat content and especially saturated fat content. Choose chicken and turkey instead of beef and pork. Good example from M&S - Thai Green Curry With Chicken & Vegetables .
Always think about how the particular dish fits into your diet and if there any benefits it brings in it. Try to increase the amount of fruit and vegetables in your diet, amount of fish and look for the options, that are the lowest in sugar and salt.
Good luck!