There is some absolutely awful advice on this thread. I very much doubt you are eating double of what you think, and as someone said it is more complex than just calories in and out. Please please don’t restrict further. Your daily diet looks pretty miserable to me, it doesn’t have to be so strict and restrictive.
The James Smith calculator is very good and if that has recommended 2000 cals per day I would be sticking to that. You can choose to stick to a consistent number a day or calculate it over the week have lower days during the week to allow for a bit of relaxation at the weekend. Whichever works best for you. Give that 3 weeks, if you don’t see a change then drop by 100-150 per day and give it another couple of weeks.
As you lose weight the number of calories you need per day will come down gradually, it won’t be 2000 forever.
You don’t need to starve yourself, be miserable and overly restrictive. It’s a long term life style change, it needs to be sustainable.
50g of porridge in the morning is not a lot and it’s bloody grim with water so if you prefer milk, have that. Or an unsweetened almond or coconut milk. Enjoy plenty of fresh fruit and vegetables.
Make sure you’re drinking plenty of water and getting enough sleep at night. Walking is the best exercise for you at the moment.
If, after 6 weeks of the above nothing has changed, I would go and see the GP.