First off congratulations on making a decisive decision to improve your life! I made the same decision recently.
I dont know if £200 will go far on the high street. By the time I bought helmet, gloves, d-lock and got him to throw the lights in free i'd still spent over £300 and had to get pump and puncture repair kit later (not that i've got a man handy to tell him to use these other things for me grr).
First thing is, it doesnt matter what type of bike you get as long as it's cool, black is always cool - mine has orange flashes. Score 10 for style, and 1 for everything else... ! This is important, you're travelling slower so might get seen!
Shimano gears: Or some equivellent, the kind you just press a lever and they do it all for you. This is essential, it's impossible to change gear and hold your phone with the other hand without a good gear selection thingamy. No seriously, you change gears often on a bike so good gears is important and saves a load of hassle.
Frame size: Get one that fits your height, you should be tiptoed if you are just sat on the saddle.
D-Lock: Get the best lock you can, don't skimp. Stealing bikes is soooo easy. If you can't afford a good D-Lock then dont buy the bike, go and nick one from somebody who made the same mistake. This is more important than drinks bottle/bag inside the frame.
Lights: LED ones are wicked bright and the batteries last yonks (well I think they do, i've never had to change them yet)
Sleeveless Denim Top with Pockets: Seriously, slip it over whatever you are wearing and always goes well, and gives you a pocket for both your smokes and your phone, handbags dont work on a bike!
Rucksack Thingy: Modern backpack rucksacky type things are dead light, waterproof, and carrying on your back rather than your bike means it's not permanently on there so you only take it when you need it, and you dont have to flaff around loading the bike up from short-lived carrier bags when you leave the shop in the pouring rain.
Racing Drivers Wet Weather Suit: Goes over whatever you are wearing and keeps all but your feet bone dry. Still working on a solution for the feet...
Sun Glasses/Glasses/Reactorlights: Essential, if you bike looks cooler than you do then you loose all style points (as I did earlier going to the shops looking a right mess). Also keeps bugs out of eyes.
Cycle Helmet: It's not uncool to wear a helmet.
Gloves, Cyling Fingerless Ones: Best. Purchase. Ever. And looks cool with other outfits too!