VLM to LCY
I think the point is that the a/c that day stops in IOM does an AM peak time flight to LCY from another point of origin, then is not required during the day, so flies to IOM where is carries a few pax, generates some £££ and can park cheaper than it could at LCY. In the evening it reverses the process.
Yes, the a/c could easily do another IOM-LCY-IOM rotation instead of sitting on the ground, but the high yielding traffic doesn't want to fly mid day. To get that traffic the a/c would need to start in IOM at 7am-ish and finish at 9pm-ish, so something else would have to give in the VLM network to generate this a/c. Then they would need to night stop a crew etc, with all that means.
Whether the attraction of a clear run at all LCY traffic would entice VLM to do this, time will probably tell.