You can't prove your theory. What we know leads us to think that the NTF designed the Iceni from scratch.
You don't get the point. Let's take an example. The F-35 Lightning II, aka Joint Strike Fighter. How long have they been designing it, and how many have already been built? Exactly.
Now let's take a SPACESHIP, with MUCH MORE ADVANCED technology, and at least 50 TIMES BIGGER. And you're saying it could be developed AND built by the NTF in maximum 18 months? No way. There are two possibilities:
1. Bosh designed the Iceni himself, and he built it using GTVA money. The GTVA would want to know what it's paying, so they would know what kind of ship Bosh is building. Only then (which is VERY unlikely) do the Terrans realise they don't have any serious anti-capship vessels in development, and they would start planning to build more Iceni's.
2. The Iceni was a secret GTVA prototype for a future battlefrigate, Bosh stole it and finished it off.
Either way, the GTVA would have built more Iceni's. The only other Terran ship with serious anti-capship capacities is the Orion, and it's becoming increasingly outdated. The Colossus can't be everywhere at the same time, so that one doesn't count either.