I'm not sure I fully understand how Apple manage the tax, but as far as I'm led to believe, all you need to do is fill out the Tax Information and agree to the contracts at iTunes Connect and that should be it.
I believe that Apple will take the required tax from the revenue before taking their 30% cut. In all it works out roughly around 50%.
Edit:
If you login to iTunes Connect and navigate to the Contracts, Tax & Banking Information section you will see a list of available contracts that you can enter into with Apple. You should see a contract for Paid Applications. If you check the box next to the contract, and click submit, you will be able to read the contract (you won't be accepting the contract by clicking submit, so this is safely a no-commit situation).
The contract states how much Apple takes for each different region, and what they do with Tax for each different region.
In the UK, I interpreted the contract to state that Apple will take and remit taxes for the UK (among other regions), as well as taking their "commission" as they call it (the commission being the 30%).
Hope this helps.