The world is short of qualified Geologists. Wherever you degree is from, you will have no issues getting a job.
Having a degree from Imperial is more likely to lead you to research rather than actually working in the industry, but either degree will be worth having. So - honestly - go to Plymouth. Its a great Uni, living in the West Country for someone interested in Earth Science/Land Formations etc will be great, and you will have a fantastic student experience. Taking a gap year / resitting - revising the entire syllabus, doing exams, up against next year's increased number of applicants at Imperial, maybe not even getting an offer from Imperial. Go to Plymouth.
Enjoy Freshers,