After reading several comments on this I spoke to training centres and they are all offering In-house on the job training,
In House meant arranged by the training centre, on the job meant at a customers property. So In-House, On the job training is possible.
The assessing body state that it should not be at the training centre, yet it does not state that the training centre can not arrange it at a different address. If they bought a house and installed a boiler, cooker, fire and water heater in each room, theoretically you could do all assessments at this property, would look a bit suspicous but not impossible.
Sorry bad wording, in house as in they have Gas safe registered engineers that you go out with and they sign your work off for you, costs an extra £1000.
I then clearly explained that these companies offer on the job training for a fee, very good way of trying to confuse OP so that people find it so confusing and don't bother, but not very informative in my opinion.
If the OP wanted to, he could quite simply go to training centre, pay the money and be gas safe registered.
Whats required to be gas safe registered
1. Training
2. Experience
3. Assessments
This doesnt neccesarily mean you will pass Gas safe registration but you will be able to apply for it.