Javelin, if you do what you say at LIFFY then you could end up in the brown stuff.
The standing agreement issue has been talked about loads before on other threads. Although they are restrictive on you guys, they are there to facilitate the flow of the traffic to aid everybody.
In your example, the ATCO at Shannon Centre has told you to descend to FL 270 by LIFFY. This gets you beneath Lakes airspace and into Manchester Area Centre's IOM sector. The Shannon ATCO will monitor your rate of descent & if it looks like you are going to make the S.A then he will transfer you to the IOM ATCO.
Now here is the tasty bit.
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.
Best scenario - the ATCOs co-ordinate your descent if it is safe to do so.
Middle scenario - you may have to make an orbit in your present position to get the height off to make the standing agreement.
Worst scenario - there is a Mil Crosser 10nm east of LIFFY co-ordinated thru at FL280, because you should be at FL270 by LIFFY. Could be messy.
If in doubt, and the same goes for anything else, ask the ATCO.