There is almost no winter tyre reference that matches rear summer tyres for M135i (245/35/18).
Technically speaking there is no reason reason 225/45/18 (which is the official size for winter tyres on that car) could not fit. It is within the supported range for 8'' rims in ETRTO Standards . My dealer is telling me the same story and I requested that they explain what is the issue. So far I am still waiting for an explanation.
I guess the main point is that officially M135i is supposed to have 4 identical 7.5'' rims for winter tyres and a mix of 8'' (front) and 7.5'' (rear) with summer tyres. So more or less we are asking them to get out what is proposed by the factory. Not sure most of them are ready to do that.