Hi OP,
Good for you doing this. I can 100% recommend your plan on doing HIIT and strength training. It has transformed my life and I love it. I was 5'4" and 12 stone. Am now down to 10.7.
Argos do the best weights. They are moulded so very comfortable and don't loosen with each move like those modular weights do. Also the best price.
If you get a kettlebell, try Kettle bells with Amy on Youtube. Free, amazing, all body workouts for 8 minutes. Do that for 2 weeks with an 8kg kettlebell and you will see the muscle ridges forming as well as feeling so much stronger.
I find dieting hard so prefer healthy eating. If you tend to eat big portions I strongly recommend filling up on stuff that is high fibre low cal. E.g. for the porridge breakfast, measure out just 30-40g of oats - which isn't much, and cook it with a big cupful of frozen blueberries or raspberries, a teaspoon of vanilla essence (which tricks you into thinking it's sweeter) and half water, half milk. That will give a big bowl of fruit porridge without the calories of extra oats.
With spag bol, make the sauce with a carrot, large onion and two celery sticks to start a soffritta, then add red, yellow and orange peppers, a courgette or two and a whole pack of chestnut mushrooms as well as beef and tomatoes. That way, you can have a lot of sauce without it being too rich. Choose big pasta shapes like rigatoni so that your bowl gets full with less pasta in it.
With other dinners, look to have at least three veg - peas, carrots, broccoli, spinach, chard, cauli etc - at least 150-200g of each - before you add carbs to your plate.
Drink lots of water. Infused with mint, cucumber, orange, lime, berries etc if you like. It does help stop overeating.
For snacks, pop your own popcorn with no oil and sprinkle with paprika, garlic granules (if you like) and sea salt. Or have a big bag of crunch salad on the go.