I would go further and say that it was at least an offer or invitation. Let's = Let us, (stop giving money to EU and give it to NHS).
The offer is clear. The acceptance was voted.
The tenets of basic contract law, intent, offer and acceptance were met.
Just a bit of a shame that the offer was not truly feasible as the numbers were wrong.
Even more shameful that the people making the offer jumped ship as soon as the contract was signed with a vote.