Article 12 of the Protocol on the Privileges and Immunities shall apply in the United Kingdom in respect of members of the institutions, officials and other servants of the Union of any nationality, including former members, former officials and former other servants, if those members, officials or other servants entered the service of the Union before the end of the transition period, provided that the persons concerned are liable to pay tax for the benefit of the Union on the salaries, wages, emoluments and pensions paid to them by the Union.