There will no doubt be many people who have certificates from other associations who would want to get a SNLP certificate. Is this possible?

The short answer is yes. The slightly longer answer is yes, if you can demonstrate that you meet the SNLP certification criteria. That of course takes time and therefore involves a certification fee.

Criteria can be found here https://snlp.uk/index.php/certification and certification takes place in the form of a 30 minute assessment session followed by a feedback Q&A, about an hour in total.