As eckhard says, 'on transition' was replaced with 'on conversion' a while back, however a lot of UK controllers will still use 'on transition' (old habits die hard...like some will still use millibars instead of hectopascals).
Anyhow. There is a lot of misinformation amongst controllers about when a/c are flying on what speed. I imagine what happened here is the controller wanted you to fly 250kts when you converted from flying on a Mach Number. We are taught that is usually around FL290 when descending however understand that it can vary greatly depending on a/c type/FMC and profiles etc. Therefore impossible for us to know when you will convert. In the UK we can see what IAS you are flying from the Mode S, however it does not show what that is as a Mach Number and which you are currently flying. So it's very difficult for us to know if you have converted or not.
In the future just say to the controller that you haven't converted to IAS yet and will do when you convert. Or if you're able to fly the IAS straight away then do so as this will show on the Mode S.
Hope that makes sense.