Regarding FIRs, a flight plan would be required for this route since it started in the Scottish FIR EGPX and ended in the London FIR EGTT.
I recomed that you take some time to consult the PPL confuser!!
A flight plan is required for a flight that crosses an
international FIR boundary. The London / Scottish boundary is not an international one.
The Scottish / Shannon one is and so is the Shannon / London one.
My question assumes that the flight was operated perfectly but for some reason chose to remain within the UK FIRS which causes and extended routing, a need to fly over high ground and a need to use a turn point in the middle of the Irish sea on an extended over-water leg.
Why not simply fly VFR (or IFR) on the direct route over low level terrain and a shorter sea crossing?