In a bid to fit everything into my and my parent's luggage, I ended up overpacking for my life here. Here's a guide for everything you most definitely should and shouldn't pack, from a girl who got half of it right.
Documents
If you can only carry one group of items, documents should be it. Your passport, driver's license, acceptance letters, loan documents, accommodation contract, bank documents and card, insurance documents are all extremely important. You should have them all together in one place and in easy access while travelling here. Keep them safely once you settle into your accommodation so that you know where to look if and when you need them.
Electricals
Your laptop, mobile, chargers, extension cords, adapters, USB's, headphones, and camera might be the items you would want to carry with you. But gaming consoles, speakers, printers, and the like would want to take a backseat. Of course, everyone has different essentials in their lives, but try to justify why that particular item should take up your precious luggage space.
Stationery
It would be a good idea to have a pen and a notebook with you for initial days but don't feel the need to carry every piece of stationery you would ever need with you, especially if you're pressed for space. You can always buy more as and when the need arises.
Kitchenware
Again, with a few cutlery and crockery pieces, a recipe book, and few other utensils you should be good to go. Other heavier items and containers can be bought once you arrive. Cross-checking with residential services about which kitchen items are provided by your hall will help you decide what to bring and what not to bring.
Nonetheless, toasters and kettles can be left behind, especially if you're in student accommodation you'll end up sharing a lot of kitchenware among your flatmates. I'm sure you won't want to end up with three toasters in one kitchen.
Bedroom
For a comfortable first few weeks you might want to bring with you some bedding, pillows, and comforters. Although some halls do provide them so it might be worth checking with them. If you prefer to bring your own it would be a good idea to confirm the bed sizes first.
Bathroom
This one is really personal but generally you should be carrying your toothbrush, toothpaste, body wash, shampoo and conditioner, towels, hairbrush, razor, sanitary towels so that you're not running around looking for essentials during your first few days. A wash bag would be essential for those who have a shared bathroom, especially if it's at a distance from your room.
Clothes
I cannot stress enough how important it is to pack according to the weather. If you are coming from a tropical climate you might want to leave most of your clothes behind. Trust me, I carried them all and didn't even end up wearing a quarter of them. But apart from that, you might want to carry any occasion wear, swimwear, suits, dressing gowns and slippers, as well as underwear, socks, shoes, and casual wear.
Healthcare
Bring a small first aid kit (pain relief tablets, cold and flu medications, band-aids, antibacterial lotion etc) and any prescribed medications. You might want to bring multivitamins and birth control too.
Miscellaneous
You might want to consider hair straighteners, photographs, makeup, your local food and snacks, religious text, water bottle and anything else you generally use.
While this isn't an exhaustive list, you should have most of the things you will need in the initial days of your stay with the items mentioned above. If you still feel dubious about this, my suggestion would be to pack light. This would ensure that you're not carrying any surplus items.
Being in London, everything you need is easily accessible and once you are settled in you can buy the things you need with your flatmates or roommate.