Was Boris Johnson's move to prorogue parliament legal?

Boris Johnson was in breach of the British constitution

Question: Was Boris Johnson's move to prorogue parliament legal?
Position1 of 3›: No, it was illegal
Argument1 of 3›: Boris Johnson was in breach of the British constitution

The constitution does not allow a Prime Minister to prorogue parliament for the purpose of stymying debate and bypassing the democratic process.

The argument

Parliament is a central pillar of the British legislative process as enshrined in the British constitution. Boris Johnson may prorogue parliament to end the session. However, using prorogation as a mechanism to bypass the democratic process and push the country into a no-deal Brexit is a clear violation of the British constitution.

Context

The prorogation of Parliament, as laid out in the laws and conventions that make up the British constitution, is designed to be used to bring an end to the parliamentary session and outline a new legislative agenda. Because Boris Johnson used the prorogation to stymy debate, it was in breach of the constitution. The primary function of the prorogation had nothing to do with outlining a new agenda and everything to do with stifling democracy.

Premises

[P1]The British constitution establishes parliament as a central pillar of the legislative process and limits the legitimate use of prorogation to ending a session. [P2] Boris Johnson used prorogation not for ordinary session purposes but to bypass parliamentary scrutiny and force a no-deal Brexit. [P3] Using a constitutional mechanism to circumvent the democratic process violates the constitution. [C] Therefore, Boris Johnson's prorogation of parliament was illegal as a breach of the British constitution.

Counter-arguments

Britain does not have a codified constitution. The government is controlled and overseen by a litany of laws and customs that dictate parliamentary behaviour and proceedings. There is no constitutional mechanism that says the prime minister cannot prorogue parliament for as long as he or she deems fit. Without a codified constitution, Johnson is well within his rights to prorogue parliament, order the Queen to veto anti-Brexit laws, or even invent new public holidays in the run-up to October 31 to prevent parliament from sitting.

Rejecting the premises

[Rejecting P3] Breaching constitutional norms is not illegal. There is no codified British constitution. Therefore, there are very few formal limits on executive power.