I think the contract was made the minute I ordered the product, the date of the parcel sending. or transit date or whatever has nothing to do with it. The contract was made under UK still in EU, so those vats/taxes/duty or whatever should apply. I think it would be unfair if they did charge duty on things that was ordered even on 31st of December.
Unfortunately not, the T&C's for OCUK cover this point:
1.3 No contract exists between you and the Supplier for the sale of any goods until the Supplier has received and accepted your order and the Supplier has received payment in full (in cleared funds). Acceptance of your order will be communicated to you by way of email at the point of dispatch of the products. Acceptance will be deemed complete and will be deemed for all purposes to have been effectively communicated to you at the time the Supplier sends the e-mail to you (whether or not you receive that e-mail).
1.4 By way of clarification, an acknowledgement of your order will be sent to you via e-mail when you place your order, but acceptance of your offer to buy the goods will not take place until the Supplier has shipped your order.
The shipping date will be your tax point and that is what will be used to determine the VAT/duty etc. at the border.
... but I really don't think you have to worry about the VAT, you are only going to pay VAT once whatever happens.
For the duty, I've not seen a definitive list of the duty rates your country will apply, so yes, that is a risk.