Do you think that PPP has any effect on the differential between German and UK schools?

Good question. Higher taxation?

I don't expect that purchasing power has a large effect there (at least compared to the typical PPP situation with India and China).

Rather I expect the following reasons:
- the fall of the pound against the euro, in the time after Brexit, this leads to salary differences which are denoted in USD

- the substantially higher number of international students in UK, esp. at LBS, compared to the German schools: LBS has 97% international students (according to FT MiM Ranking 2017, versus e.g. the 27% of WHU), which means only 3% are British nationals. While this not directly leads to a salary difference, it can be argued that foreigners face typically a difficulty in the job market, which could impact the sales figures

- the German MSc degrees typically require a Business undergrad degree. This implies that the people in Germany have earlier on focused on potential jobs and potentially done more internships in the relevant field

- the German MSc courses are two-year courses that enable people to do an internship and with that to improve employability, whereas most UK programmes are one-year programmes. For instance, FT MiM 2017 ranking states that 50% of LBS students did an internship (as I understand in the course of the programme) compared to 100% of WHU students.

- Salary difference in Consulting (for other industries I don't know): Consulting is a typical pathway after a MSc (especially at the German schools in the FT ranking). I know of several instances, where the pay in Germany is (admittingly before taxes) higher than in London. Several employers in Germany hire people with a Masters degree to a higher position/rank than people from a Bachelor degree, whereas my impression is that in the UK, there is no difference between Bachelor and Master in this regard. I know of instances, where a person starting in London with an Oxbridge MBA gained at the same firm the same rank as a person starting in Munich with a German MSc. (not to speak of the cost of living in London)
Maybe this is also question of labor supply and demand. The German speaking region (DACH) is the world's second biggest consulting market after the US, but nearly all consulting firms hire people with sufficient German skills. UK and & Ireland are close, but below the consulting market size of DACH, however there are many more people speaking English and qualified for the jobs (which also shows that e.g. LBS is a more attractive target for international students than the German schools). If supply and demand hold true, than the salary in the UK should be lower by comparison.