You'll need to file form
RPPL-F-120 with IAA consenting to your licence records being shared with UK CAA.
The ATPL exams can be credited towards the PPL in accordance with point (b)(1) of FCL.035:
(b) Crediting of theoretical knowledge
(1) An applicant having passed the theoretical knowledge examination
for an airline transport pilot licence shall be credited with the
theoretical knowledge requirements for the light aircraft pilot
licence, the private pilot licence, the commercial pilot licence and,
except in the case of helicopters, the IR and the EIR in the same
category of aircraft.
The credit should be within the validity period specified in point (c) of FCL.025.
Selection and designation of an examiner for a PPL skill test is normally done by the training organisation. Alternatively you may request designation by the authority by following the procedure described in para 4.3 of IN–2016/004. I don't know whether you will be permitted to attempt a skill test without first being recommended by a training organisation. That's a question for your examiner or try the
Flying Instructors & Examiners section of this forum.
Don't forget to arrange an exam for the initial UK medical certificate.