The UK Institute for Government has helpfully compiled the following graphic. You will notice that no UK Brexit bill is yet enacted. Nonetheless, unless otherwise agreed between the UK and the EU, from 12 midnight CET on 29 March 2019, the UK is a third country (in EU law terms).
In the event there is agreement between the UK and the EU on Withdrawal, there will be an additional Withdrawal and Implementation Bill (not to be confused with the Withdrawal Bill that is presently in progress). It is this Withdrawal and Implementation Bill that may set out the provisions for the Transition Period (if such is agreed). Attached to this Bill, may be an outline of the future relationship between the UK and the EU. 
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