@MaybeNotBob
Cameron was the PM who offered a referendum. Remain campaign issued the booklet to every household in the UK which included the statement, with my emphasis underlined.
This is your decision. The government will implement what you decide.
Delivery is the responsibility of the person who made the offer. Not the person, or persons, who accepted the offer. For example, I place an order for a car, it’s the duty of the dealer to deliver it, not me.
The outcome of the referendum was set aside by the Miller case. This required the decision whether or not the UK should leave or remain in the EU had to be made by Parliament. 498 MPs voted to leave. 114 voted to remain. The 498 were mostly Conservative and Labour.
So, logically each of the 498 should have had a plan on how to leave the EU.
The general election on 8 June 2017 was 42% conservative and 40% labour. Both parties committed to honour the decision to leave the EU. That so, they too should have had a plan.
https://www.cep.eu/eu-topics/details/the-uk-governments-brexit-white-paper.html
Is in one of the several white papers for UK strategy for leaving the EU.
However, Labour went back on their word and switched to being a remain party in the hope they could force and win a general election.
So, if you are looking for people to blame, I wound say the order would be
1st
Cameron making an offer without a delivery plan.
2nd
Labour for changing their mind in 2017.