based on emissions data I could find on google, you’ll be taxed on:
Merc - 25% of the car’s P11D value
Astra - 27% of the car’s P11D value
Octavia - 25% of the car’s P11D value
This is the table you need. Bear in mind the premium you pay for diesel (right hand column):
The merc is, in fact, the most prudent choice here