What I am sure of is that the transition level is the lowest usable flight level and as such must be above the transition altitude, hence the transition layer
What you said in the above quote is 100% correct.
However, in the example - Transition Altitude 3000ft QNH 1010, the lowest usable flight level in the UK according to the Quadrantal rule is FL35 and the transition layer is 404ft thick.
If the QNH was 1045 Milibars then FL25 would find itself some 3460ft AMSL and thus would be the transition level in the UK and would give a transition layer of 460ft.
As was said previously, the transition level varies depending on both the transition altitude and the QNH.
Regards,
DFC