Derek says that Brexit has nothing to do with a weak £ so therefore it must be true.
Personally I think the major fault in your argument is the tacit assumption that Mrs May is bargaining from a position of strength. She simply isn't. If she was then we would have been able to renegotiate a better deal when Cameron tried and there would never have been a referendum in the first place.
Looking at the trade imbalance I believe she was bargaining from a position of strength. Her biggest mistake IMHO was allowing the EU to dump the Irish border issue in her lap. The EU mandate says there will be a border observed between any EU state and it's non-EU neighbour so the problem is theirs and she should have refuted any responsibility. She gave them a noose to hang her with. We always have had a border of sorts in Ireland which met our requirements so we don't need a change. Even now it is not too late to refuse the backstop and throw it back in their lap. Cameron allowed the EU to treat him like a lapboy and accept the miserable offering. "Peace in our time" comes to mind. Cameron was and is a wanker.
