Ok, your diet doesnt seem too bad.
Obviously I would say drop the crisps, use wholemeal bread for sarnies and cut out the mayo.
Some wholegrain cereal's provide a great start to the day, combined with a portion of fruit. Porridge oats are excellent to as they should fill you up til lunch and is a slow release enrgy food.
Pasta is good if kept simple. Lasagne for example is really heavy and high in carbs. There are plenty of simple quick pasta recipe's on the web.
Fish and white meat should be eaten daily as your primary source of protein. Combine with pasta, wholemeal rice, veg or as part of a salad.
I only eat red meat once a week, love a good steak.
The theory of little and often is great for weight control and will stop you from over snacking during the day. Our hectic lifestyles lend themselves to convenience foods, its not always easy to eat the right stuff all the time!
I could go on for ages about diet, it is a very individual thing. Just avoid processed food and saturated as much as possible. The nutritional value featured on food these days is a good guide.
Fitness level is again, very individual. Age, weight, lifestyle all play a part. If you are pushing yourself then that is all that matters, it is relative and if you mark down your performance (time or distance) each session you should see improvements.
Your body will adapt to exercising again, it may take a while but that is why performance indicators are important. E.G even if you can swim 500m 1 minute quicker over a month, that IS an acheivement, your body IS getting fitter, Dont expect it all too soon. If you enjoy excercise, great, that is half the battle! Swimming is a great all round cardio and muscle toning pursuit. If I had to pick one aspect of training and could only do that one thing, I would swim. It will help with weight loss to try combining with some other cardio such as power walking/running.