None of the options represent my opinion!
As a fixed gear rider that barely touches my brakes, I believe that I could navigate london or any cycle friendly city with little concern for my own or anyone else's safety, at least, no more concern than riding any other bike. HOWEVER, I do run a font brake to comply with the law and for those just in case moments where a hard braking manouvre without locking a wheel is required or where the flywheel effect of the rear wheel may be difficult to resist, such as going down a steep hill.
Surely the issue is you have half the braking power of somebody with a front and rear brake?
If you lock the front wheel - game over. You're on the floor which is why having 2 brakes is better. Not to mention legal.
I don't understand why single speed and fixie riders thing it's "cool" to only have a front (or no) brake. You won't look cool when you couldn't stop yourself when a bus pulls across you because he didn't see you and you end up on a slab in a mortuary.
You don't have half the braking power of someone with a front and rear brake, you can effectively brake using your legs. Also, you are encouraged to be more aware of what is coming up and controlling your speed on a more continual basis rather than being tempted to rush ahead then brake late, accelerate hard, brake late etc.
Same on any bike! A fixed rider still has a rear brake, it is just in a different form. Rather than assuming only 1 brake is present, it would be better to acknowledge that 2 brakes are present (as the law does) but debating the efficiency of one form vs the other.
Also for your consideration, track bikes traditionally are not drilled for brakes! So there is no mount for a rear brake. For a front brake you would need to switch out the forks for road forks.
I acknowledge in some circumstances, there may be an increased level of risk, riding a fixed gear bike with only a front brake but the degree that the level of risk is increased, IMO is blown way out of proportion.