Why didn't V defend himself as soon as Peter Creedy and henchmen started shooting V?

Why? V could've thrown his knives as soon as they started shooting, and mitigated the gun wounds on V that proved fatal.

1. Given that V was wearing a steel breastplate, he wanted to orient himself in such a way to where the armor would take as many shots as possible. The torso is a much larger and easier target to hit than the head, legs, etc., and V banked on Creedy's entourage shooting just there.

2. If V would have tried throwing his knives while they were shooting, his accuracy wouldn't be nearly as good and arguably would have risked his hands/arms getting shot even more so than just standing there.

3. If V would have started running around while they were shooting, there's a good chance that some of Creedy's men could have have circled around V and he'd then be surrounded. By directly facing Creedy's men and letting them all shoot at once, V was most optimally protecting the non-armored parts of his body.

Before arriving at that basement (what's the correct term?), he could've armed himself with a gun, then use it to shoot back.

Simply put -- guns aren't V's style, they're too impersonal.

In-universe, there are indeed several problems in this scene, but:

Out-of-universe, him fighting back immediately wouldn't have the same cinematic effect. The scene isn't designed to have Creedy face off against an equal (as you would often get in action film finales), but to have him be utterly powerless against a force he doesn't properly understand. V's quote at the end of the scene sums this up:

Behind this mask there is more than just flesh. Beneath this mask there is an idea... and ideas are bulletproof

As I understand it, this is a reference to a plot point in the original source material, where the original V actually does die, and Evey takes his place wearing his mask. Thus, there really is more behind the mask than "just flesh"; the idea causes "V" to (essentially) reincarnate.

From the enemy's perspective, this would feel quite similar to what we see in that scene.

Also as others have noted, V really had no intention of surviving that night.

Beneath this mask is an idea Mr. Creedy, and ideas are bulletproof.

That's kind of sums it up. It was either a plan for selfsacrife (he made himself a martyr) or he really believed he is bulletproof.

I don't have much evidence to back this up but I presume it was to eliminate the possibility of accidentally being shot somewhere he didn't want to be shot. Most law enforcement officers are trained to shoot for the torso because it's the easiest part of the body to reliably hit and has a sufficiently high chance of fatally wounding the target. V expected this and wore a protective layer under his cloak to withstand the barrage of bullets.

But for that protective layer to work, he needed to ensure that that's where the bullets would hit him. He gambled that if he gave them a stationary target, none of them would deviate from aiming at his torso and that they were trained well enough to be able to consistently hit their mark. If he were to have attacked, he'd have become a moving target and exposed unprotected parts of his body to the incoming fire, greatly increasing the chances of him being incapacitated before completing his revenge. For example, his hands might get fingers blown off while trying to throw his knives, or he could take bullets to his back when he turned to attack a particular soldier.

So by focusing his defense on the one spot he expected them to focus on and letting them focus on it uninhibited, he gave himself his best chance at surviving long enough to do what he went there to do. In other words, he used his enemy's strength and confidence against them.