You don’t really do research in Part I or Part II and a big difference from the US is that getting summer lab positions as an undergrad is uncommon, doing so in term time is unheard of.
The traditional DAMTP Part III is a math course not an experimental physics course, although you do write a research paper (“extended essay”) for credit as part of the course.
I’m thinking more about experimental and/or lab-based courses, which come under NatSci Part III (this optional fourth year has now become common in STEM subjects, by calling it Part III it comes under undergrad funding rules so is cheaper than a standalone masters) and do include a substantial research component:
Astrophysics:
A major highlight is the research project providing you with a unique opportunity to get to the cutting edge of astronomical research. Projects often involve data analysis or a simulation and culminate in a dissertation that may reach publishable quality.
Physics:
You’ll undertake a substantial research project, diving deep into a topic of your choice alongside world-leading academics.