It's explained in the campaign:
"It is written into law that the UK will be leaving on 29 march 2019, but the deadline could be extended. As at the time of launching this campaign the UK is still part of the EU, all the UK pledges must be done with the "(EU)" reward option. If at time of shipping the rewards the tax is not aplicable anymore we will proceed to refund any charged VAT."
But if you feel more confortable to choose the Non EU rewards feel free to do it. At the end of the campaign we will contact the UK backers personally to solve each case if necessary.