Whilst you were with Zen, the DLM (Dynamic Line Management) settings for your line would have had time to enable/disable VDSL features to give you the fastest, most stable service your line could physically support.
When you change supplier, these settings are reset, so DLM needs to re-learn about your line.
At the moment, your line will be running without any bells and whistles enabled to combat interference or suboptimal line conditions.
In particular, a feature called G.INP, designed to limit the impact of crosstalkers (other VDSL users' signals), will currently be switched off.
Over the next week or so, DLM will start enabling features (G.INP, Interleaving) to increase your line's stability. It may also start lowering the SNRM (signal-to-noise ratio margin) if your line reaches a level of stability that'll support faster speeds.
As you're using the same modem as before, there's no reason why DLM won't eventually bring you back to the rates you were seeing before.