The IOM ATCO descends you to FL190, therefore the restriction that the previous ATCO gave you still applies as it is not that ATCO that has given you a new clearance. SO the clearance of FL270 at LIFFY still applies. If you do not meet the standing agreement then this causes a lot of co-ordination betwwen sectors as you will be entering someones airspace that you are not permitted to be in.
Like BOAC says, this is not what the CAA and NATS mandate.
There is no room for interpretation really. A new clearance will cancel any restriction in that part of the aircraft's profile (by that I mean a horizontal instruction won't cancel a vertical restriction and vice versa).
Otherwise there is no cast iron guarantee that the pilots will all do the same thing and no guarantee that an ATCO won't end up with egg on his face.
If you (as an ATCO) need a previous restriction adhered to then you either have to restate it, or else give an instruction such that the aircraft's minimum performance in accordance with the rules (500fpm climb/descent rate) will meet the restriction anyway.