I am not 💯% sure, but I am retired enlisted (retired in 1988) so I don’t think they could ever recall me, but I think (not sure) commissioned officers are subject to recall, the rest of their lives. Is that your understanding too?
It's the other way around. You, as a retiree, can be recalled back to active duty at any time for the rest of your life although it's unlikely after being over 60 or retired for more than 5 years.
https://www.thebalancecareers.com/u-s-military-rules-for-being-recalled-to-active-duty-3357010
"Retirees (those who spend at least 20 years in the military and draw retired pay) can be recalled to active duty for life. However, the policy established in "DoD Instruction 1352.01 - Management of Regular and Reserve Retired Military Members" makes the recall to active duty unlikely for those who have been retired for more than five years, and those over age 60."
There is a 1,000 headcount limit although the DoD wants to increase that number. According to military.com, the US Army contacted more than 800,000 retired soldiers to inquire if "they would be willing to assist with military’s pandemic response."
https://fas.org/blogs/secrecy/2020/06/retiree-recall/ 
I don’t know what the UK’s policies are, that was my hypothetical.
As for the US enlisted, it is 8 years obligated service time. Anyone serving less than 8 is subject to recall from the Individual Ready Reserve (IRR).
For retirees, you can be recalled by your Service Secretary, per EO 13223, under the regulation of SECDEF. There are limitations, but under a National Emergency, the limitations go out the window. US Marine specific, section 688 of USC 10 spells it out.
In practice, we would never get recalled unless the US was at death’s door.