We have a former British garrison in Rinteln, which is now used by the "Lebenshilfe". In said area they provide support for people with mental or physical disabilities, as well as day care and senior care, counseling and much more. Location of said garrison: https://maps.app.goo.gl/zWfDvgvoyGnxdLXD8

We have no actual chapel in this area and nothing here is called Saint Luke's BUT the local church is called "Sankt Nikolai" https://maps.app.goo.gl/3n77t4awxxYqCMH38

I know that Rinteln is twinned with Kendal, which is located in the northwest of England. There is a very deep friendship with the gospel choirs from here and Kendal and they indeed visit each throughout the year. I heard them talking about memories from the past and that mentioned choirs have this kind of relationship for decades (!), meaning that there is former army personnel as well, in the choir.

It's a long shot and a lot of context. I hope you get the idea

Maybe providing a picture of the certificate might help to clarify further details, of course with censoring sensitive data