£800 camera kit? What to get?

I'm a 17 year old photographer currently doing A Level Photography. I'm specialising in portrait photography and wedding shots etc. I have around £800, for this I want to buy a camera body, a decent lens, and a decent tripod. What would you recommend? PS I already have a studio with softboxes etc, I just need the camera equipment as my current DSLR (Nikon D3100) is just not good enough.

Added (1). Basically my current camera, when i make a portrait bigger than A4 would be blurry and not sharp.

1. What is your current camera lacking?
2. Do you consider medium format film viable in your situation?

I can divulge my suggestion upon receiving the desired response from you.

All praise be to Eastman K.

Edit:
You have many, many options. I'm biased towards film, which is not everyone's preference, and will suggest a medium format camera suited for what you need. For a venerable studio camera, a couple of Mamiya RZ series bodies will have no problem providing you with the images you require. You should be able to use RZ (i.e. NOT RB) bodies and lenses with your studio strobes. The Revolving back on RZ bodes allow for a quick shift of orientation without much effort. There are is a large section of used bodies, lens, etc for this modular system available online from reputable sellers and within your budget.

With the appropriate scanner, you can pull massive amounts of data out of your 6x7 portrait slides and negatives, allowing for larger, more pleasant prints with minimal cropping. 20x24 or above is not at all unreasonable with this format. You can send film out to local professional labs to handle developing, scanning, and printing if you like. There are many professional portrait films available to you from Kodak and Fujifilm, allowing you to achieve the desired image qualities for your portraits.

More info about the RZ here: http://camerapedia.wikia.com/wiki/Mamiya_RZ67#Lenses

You can get an idea of RZ gear prices here (they ship internationally): http://www.keh.com/Camera/format-NA/system-Mamiya-RZ?s=1&bc=42&bcode=RZ&bco=559

All Praise Be To Eastman K