Hi
For the SEP & MEP it's training as required then pass an LPC.
Don't mean to critize the UK CAA but as per EASA a SEP or MEP rating lapsed more than 3 years requires training as per initial issue (6h dual + checkride).
Wanted to mention as a heads up to anyone with a licence issued in another country. When sending the paperwork to the respective CAA it may be rejected. I was in a similar situation with my MEP and IR and I ended up doing it at "home".
By the way, I'm pleased some seasoned aviators find their way back into "basic" flight instruction. You are certainly needed, not just by the student pilots, but also by the often overlooked junior FIs.