It is not my area but some general points will appply.

You will need to be familiar with common UK practices - depending where you have worked before that may or may not be a significant step. We do not have a licencing regime as such, but a thorough knowledge of which standards apply to what (and there are a lot of standards to consider, building regs, electrical regs, energy regs etc ) will be needed, along with some insurance. You may be better to start working as part of a larger team, so your deigns are peer reviewed, even if it only for  short time, see it as a bit of a loss-lead to get the experiance in first.

Mike.

Thank you for your valuable advice mapj1.

You mentioned that you are an MIET and working towards becoming an IEng. This is a great way to demonstrate your competence and professionalism in the field. In addition to your academic qualifications and professional registration, you will also need to have a thorough knowledge of electrical codes, standards, and regulations, such as the IEC 60364 series, BS 7671, and BS EN 50110. Also experience within an electrical building services role including undertaking calculations and producing drawings.
Ideally hold an accredited Building Services degree or higher educational qualification (HNC/HND)
Experience using AutoCAD and Revit
Experience using Amtech and Relux or Dialux