This is a whole catalogue of 'gotchas', that fortunately, everyone walked away from.
I don't know what plates the crew were looking at prior to departure, but the transition altitude at EIKY is 5000'. On all the cockpit-trips I was privileged to do during 30 years as an operational ATCO, I'm fairly certain TA was mentioned as part of the pre take-off briefing, conducted by the crews. Was this a single-pilot operation? The report makes references to 'the crew'.
If I remember correctly, the 'FL One hundred' phraseology was brought in in the U.K. FIR due to the frequent confusion between FL100 and FL110.
It can be argued that similar confusion is unlikely between FL200 and FL210/300 and 310, etc.
The proposal for a harmonised EASA Europe-wide, transition altitude is also interesting. There is no doubt that some form of 'standardisation' is required. In the U.K. FIRs, we have TA's of 5000, 6000, or 3000', depending on where you are.
My own personal preference would be a TA of 6000' across the UK/Ireland FAB. OK, it doesn't align with the terrain in the rest of the EASA/SESAR region, but if you're flying over the U.K. or Eire, the highest thing you'll be likely to hit is Ben Nevis, at 4414' AMSL.
One of the problems with introducing an 18,000 transition altitude in the UK and Eire is the frequent occurence of steep horizontal pressure gradients, which are related to our position on the edge of a North Atlantic continental land-mass. Descending from FL 200 the QNH at the transition point could easily be 4mb/HPas different from that at the destination airfield. i believe the Met Office have suggested upwards of 40 altimeter-setting-regions, (ASRs), would be required. If 'climate-change' predictions are to be believed, such steep pressure-gradients will become more frequent in the years ahead.
Safe flying Y'all, and as has been already said..."If in doubt, just ask". Anything which prevents 'form-filling' is good news, no matter which side of the mic you're on.