Most Bucks schools are still using levels as no 'official' guidance has been issued and they prefer to use something they, and parents, understand.
They will get short shrift from Ofsted if they aren't at least working towards a new system by Easter. The guidance is clear, levels are not to be used any more - any school ignoring this is on very dodgy ground.
However, your school seems to be on top of this though, MrsG, and are right that the criteria have changed and are more challenging for children in the current Yr 5 and below. When previously a child in Yr 6 may have acheived a level 5 and be deemed to be working 'above expectations', the same child doing the same work may now will 'only' be acheiving expectations and not exceding them!
It's a nightmare, and as you say, how this will pan out in the future when progress has to be compared and proved, is anyone's guess ... Ho hum - as long as the politicians are happy, eh?