Noel,
The UK Pressure Systems Safety Regulations (that SI 2000 No. 128) is still in force. It had nothing directly to do with EU regulations. It isn't a construction code though Regulation 4 relates to Design & Construction.
Regulation 8 relates to the Written Scheme of Examination.
The EU Pressure Equipment Directive, has been transferred into UK law by The Pressure Equipment (Safety) Regulations 2016, as amended by the Product Safety and Metrology etc. (Amendment etc.) (EU Exit) Regulations 2019. So it essentially remains in place, but with UKCA mark replacing the CE mark.
In the UK specific codes are not mandated – you can work to any code or standard as long as you can show it satisfies the Essential Safety Requirements laid down in regulations.