Best bang for the buck to improve a cheap fixie? What are things to improve on a cheap bicycle that make a real difference? I'm looking at the bang for buck tradeoff. 

The bike is an inexpensive Fixie/Track Bike (less than 150GBP). 


11 kg steel frame
Flat bar
Front and rear V-Brakes
In that order (my opinion, of course):


Tires. Bad tires suck your energy and are prone to flats. Good tires can make your bike fly, sometimes even act as a suspension. As an extra, tires aren't actually part if the bike, so if you have good tires, you can use it in other bike(s);
Brakes and brake levers. This is safety and comfort. Bad brakes can make your hands sore, if you have to squeeze them too much to stop. Good brakes give you much more control of the bike than anything else. Good brakes are THE things that allow you to go really fast if you want/need.
Saddle and grips. These are contact points on the bike. In the end, all the bike-rider interaction flows through them (and the shoes you use to pedal, but they are not part of the bike). These parts alone can turn a very nice ride into a very painful one, and vice-versa.


I think if these are OK, it's possible to live with a relatively cheap bike, provided it fits and don't require too much mainteinance, which are also required for a nice biking experience.