Until the end of 2022, placing goods on the market in Great Britain (England, Wales and Scotland) is still possible with the CE-marking; by 2023, the UKCA procedure will be required exclusively.
Manufacturers have to go to a UK Approval Body and have to apply for a UK CA-marking. Guidance for using the UK CA marking can be found at: 
https://www.gov.uk/guidance/using-the-ukca-marking
It is up to the UK Approval Bodies whether and which ATEX-documents are taken into account as part of the UK CA-procedure. IECEx documents are to be accepted by UK Approval Bodies, which are also members of IECEx, according to the IECEx rules; but certainly subject to a review of the IECEx documents. (Similarly, the other way around also applies: Documents from an UK Approval Body that are to be used as a basis for an EU-Type-Examination Certificate of the PTB will be assessed individually and in individual cases a re-evaluation or even a new test may be required).
Comparable rules to ATEX apply, therefore a UK Approval Body must be listed in the UK. The PTB is and will not be listed there.
Accordingly, it is not possible for us to commission a UK CA-procedure.