In order to get any reduction in flying hours on an integrated course it is necessary to hold a licence. Therefore The Butler would not get any reduction at all on his present 25 hours. With a PPL it is possible to get up to 50% of one's flying hours as a reduction from the syllabus subject to a maximum of 40 hours (45 if you hold a night rating).
Irrespective of any reduction in training, anyone starting at an integrated school would still have to do a certain amount of flying before being sent solo. The average seems to be around 6 hours. The reason is that, to the school, that student is an unknown quantity when it comes to his flying ability which therefore needs to be assessed. In addition, merely holding a licence is not any guarantee of currency, nor is it any indication of the standard of training previously received. Moreover, in the case of overseas schools, the student would need time to become familiar with local rules and procedures (plus a different ATC environment).
The reason that people rarely get 50% off is due to the structure of an integrated syllabus. Of the 150 hours of flying, a minimum of 50 hours must be SPIC. 50 hours of solo navigation must also be logged , along with 5 hours night flying and around 30+ hours of twin engine time. Throw in the various tests (Progress Tests, CPL Skill Test and IRT), and there is very little scope for offering the full 50%.