Article 14 Independent Personal Services
(1) Income derived by an individual who is a resident of one of the Contracting States from the performance of personal services in an independent capacity may be taxed in that State. Such income may also be taxed in the other Contracting State if:
(a) the individual is present in that other State for a periods exceeding in the aggregate 120 days in the tax year concerned, but only so much thereof as is attributable to services performed in that state; or
(b) the individual has a fixed base regularly available to him in that other State for the purpose of performing his activities, but only so much thereof as is attributable to services performed in that State.
They are anticipating your being in the country. From what I have found most will withhold even prior to the proper permits.
Article 22 is used for Elimination of Double Taxation.
(1) Subject to the provisions of the law of the United Kingdom regarding the allowance as a credit against United Kingdom tax of tax payable in a territory outside the United Kingdom
You can claim the credit on UK filing.