I notice that we rarely use model codes for more recent Opel/Vauxhall vehicles. Since the 1980s, I believe since the merging of the Opel and Vauxhall ranges, and until the PSA takeover, there are precise model codes for every vehicle produced, including the one provided by other carmakers in joint-venture. Following ones I could find so far:

Adam: M13
Agila A/Mk1: H00
Agila B/Mk2: H08
Ampera: R12
Ampera-e: F17
Antara: L07
Arena: A97
Ascona C/Cavalier Mk2: J82
Astra F/Mk3: T92
Astra G/Mk4: T98
Astra H/Mk5: A08
Astra J/Mk6: P10
Astra K/Mk7: B16
Calibra: C89
Cascada: W13
Combo B/Mk2: S93
Combo C/Mk3: F25
Combo D/Mk4: X12
Combo E/Mk5: K9
Corsa A/Nova: S83
Corsa B/Mk1: S93 (including the South-American-sourced wagon)
Corsa C/Mk2: X01
Corsa D/Mk3: S07
Corsa E/Mk4: X15
Crossland: P17
Frontera A/Mk1: U92
Frontera B/Mk2: U99
Grandland: A18
GT (2007 model): M07
Insignia A/Mk1: G09
Insignia B/Mk2: Z18
Kadett E/Astra Mk2: T85 (including the Combo/Astramax)
Karl/Viva: C16
Meriva A/Mk1: X03
Meriva B/Mk2: S10
Mokka A/Mk1: J13
Monterey A/Mk1: M92
Monterey B/Mk2: M98
Movano A/Mk1: X70
Movano B/Mk2: X62
Movano C/Mk3: U9
Omega A/Carlton Mk3: V87
Omega B/Mk1: V94
Senator B/Mk2: V88
Signum: Z03
Sintra: APV
Speedster/VX220: E01
Tigra A/Mk1: S93
Tigra B/Mk2: X04
Vectra A/Cavalier Mk3: J89
Vectra B/Mk1: J96
Vectra C/Mk2: Z02
Vivaro A/Mk1: X83
Vivaro B/Mk2: X82
Vivaro C/Mk3: K0 (including Zafira Life)
Zafira A/Mk1: T98
Zafira B/Mk2: A05
Zafira C/Mk3: P12