Back in the early 1980s, the SIDS from EGCC did go to Flight Levels. I still have the charts in the loft somewhere. In fact EGCC had SIDs which finished in 4 different countries.
HONILEY/LICHFIELD/OTTRINGHAM
BRECON.
DUBLIN.
TALLA/DEAN CROSS.
Actually 4 1/2 if you count ISLE OF MAN departures.
Prior to the alphanumeric designation system, the SIDs from R/W 06 were called '61' departures, and those from R/W 24 were '60' departures.
The transition altitude at this time was 4000ft, and the OTR 60 SID passed underneath the Barton stack, climbing initially to 4000ft.
It all worked splendidly until a chap in a TU54 departed on an OTR 60 and climbed to FL60, narrowly missing someone who was holding at FL50 over the BTN VOR.
Thread drift? Certainly. So, back to the 18,000ft UK TA.