Full Fact have a good breakdown here:
https://fullfact.org/europe/our-eu-m...ee-55-million/
I suspect the case will hinge on the claim we 'send' £350 million rather than we are 'billed' £350 million or the EU membership fee 'costs' £350 million send we literally do not send that amount at all. I think it was a lie and it's a bit silly to argue it isn't, no one
truly believes that the claim was talking about a hypothetical amount especially since the claim was that the money would be available for the NHS which is clearly would not.
But to me this is standard politics. The Government also were involved in 'creatively' using numbers when they claimed it would cost £4,300 per household to Leave:
https://fullfact.org/europe/4300-que...usehold-worse/
I don't think this case will get anywhere and there are troublesome consequences if it did. If this case actually goes ahead then you would every election litigated after the fact by the losing side.