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.
Parliament has never been prorogued for as long as Mr Johnson proposed, leaving it without legal precedent.
Boris Johnson’s prorogation is far longer than any that preceded it, leaving it without legal precedent.
Boris Johnson's decision to prorogue parliament on September 12, 2019 was unprecedented in British political history. The new parliament wouldn't start until October 14, a full 33 days since the previous parliament ended. Not since 1931 had a parliament been prorogued for 33 days. Since the turn of the millennium, the average length of time between parliamentary sessions was 8 calendar days.  Boris Johnson's break with convention makes his prorogation unlawful. Because the British constitution is not a written, codified document, parliament and prime minister is bound by convention. Any breach of convention must be seen as an unlawful act.
The parliamentary session running from June 21, 2017, to September 9, 2019, was the longest on record. It ran for 810 calendar days. To allow it to continue would have defied precedent and pushed parliament further into unchartered waters.  If, as Johnson's critics claim, convention is essential to the functioning of government, then they should celebrate the prorogation, not criticise it. Secondly, the length of the prorogation was also not unprecedented. Much of the period between sessions was already covered by the three-week parliamentary recess already planned for the party conferences. If you take this into account, the actual length of the prorogation was only seven days, not 33. This is a perfectly normal period of time to have between parliamentary sessions.
[P1] Breaching parliamentary convention is unlawful. [P2] Boris Johnson's prorogation is a breach of parliamentary convention. [P3]Therefore, the prorogation is unlawful.
[Rejecting P2] Boris Johnson's prorogation was not a breach of convention.