Starting up as a new owner operator and working out costs

I think what you’re suggesting is costs over 12 weeks is about £18k. Now add your PMIs, the odd tyre / fault, days off driving due to these. Assuming you work 6 days, that’s about £250 a day, before you make a penny.
Are you comfortable with your estimates? Can you do the full driving time and the accompanying paperwork & looking fir the next job?