Instead of asking for a transition period prior to any Agreement the UK should ask the EU do they want a comprehensive free trade Agreement with us or not. If no, we should just get on and leave in March 2019 under the WTO option. We would pay them nothing and be able to put in our own border, fishing and other policies immediately.
If the answer is yes then set a deadline to sort it out this year and see what else we might agree to in return. Nothing is agreed until everything is agreed. There is no point in paying them lots of money for a worse deal than the WTO option. Nor should they stop us getting on with negotiating free trade agreements with others over the next year to be ready for April 2019.