250kts, just for info, from my recollection (for what it's worth), I have never heard "expect so" from my time there. But, a younger generation of controllers (from many nationalities) work there and it's possible that one or some use this odd phraseology! When it was known/planned that higher was available a little further on, generally it was quite simply "maintain FL???, you may expect higher". Again, depending on the individual, some would add, " in X minutes" or "with the next sector" etc. The requested FL would never be mentioned by the controller unless in an actual clearance.