It is an odd statement !
An endorsement on a driving licence usually means that penalty points incurred by the driver for a driving offence are recorded on that licence.
However, if you have no penalty points on your licence, I would assume that *endorse* in this instance means a confirmation that your EC licence is valid in the UK.
I always understood "endorsement" to mean "approval of" or was some way of adding comments.
You've got me scratching my head now too - no wonder you are baffled !