As well as NATS, the other two independant CAA Approved Training Centres in the UK (Resource Training and Global ATS) provide OJTI Courses.
However, in the UK, you cannot undertake an OJTI Course unless you have a UK CAA Licence, have completed the requisite operational ATC experience (usually 2 years after the first validation - this can be reduced), have been recommended by your own Airport Management AND the Regional AATSD CAA Inspector has approved your presence on the course.
You will not be allowed to attend the course as a self sponsored candidate.
I think it's the same throughout Europe as well.