Plumbing – How to block/seal a low pressure water pipe

Many posts to this forum are about how to clear pipes – this one is about how to block one!

I have a cold water header tank in my attic, there's a short length of pipe that feeds from this tank to the hot-water tank in the airing cupboard below. The pipe is (I think) steel and appears to be BSP threaded 1 inch pipe. The pipe is corroded, and attached to a brass gate-valve that's been seized for many years. The pipe then converts to 22mm copper pipe before getting to the hot-water tank.

I'd like to replace the seized gate valve, and since I'm comfortable plumbing with copper pipe the simplest option (for me) seems to be cut a new hole in the tank, fit a new 22mm compression tank-connector, and then re-plumb from there to the hot water tank.

The old tank connector appears to be steel and is very rusty, I'm also pretty sure that any attempt to remove it to re-use the hole would fail, and would probably damage the tank in the process.

Bearing this in mind, I'd like to cut-off the existing pipe 6 inches from the tank and simply block it up. What's the best way to do this? I've not worked with steel pipe and don't have access to threading equipment. Since the pressure at this point is equal to the height of water in the tank, it's doesn't need to be a very sophisticated solution. Is there a reasonably reliable of blocking such a pipe?

Thanks