Hello there!
Congratulations on your offer!
Wherever you choose to study medicine, the training will ultimately prepare you for foundation level in the same way. Both Imperial and Bristol have amazing courses, offer excellent opportunities for research, quality improvement projects, and other experiences that will strengthen your future applications. I don't think there’s a major advantage of one over the other when it comes to opportunities.
Another point to consider is that, although your undergraduate medical degree plays a role in shaping you as a doctor, it’s actually your postgraduate training — such as the Foundation Programme and beyond — that tends to have a greater influence. The differences between graduates from different medical schools become much less noticeable fairly soon after qualification. While there is some evidence that graduates from certain medical schools perform slightly better on exams like the MRCP (Membership of the Royal Colleges of Physicians), this is not necessarily a true measure of being a "good doctor."
The key is finding a medical school where you feel supported throughout your studies and where you can thrive over the years ahead.
Also, it’s worth bearing in mind that receiving offers one year does not guarantee offers in future cycles. Competition for medicine is becoming increasingly fierce — both at the university admissions stage and in the job market after graduation. While I hope that by the time you reach that stage things will have improved, it’s something to be aware of as you make your decision.
One thing that you may need to consider, is it the cost of medical school is a significant factor. I can understand if this is a significant challenge for your choice. If this is the case, it would be worth reaching out to the medical school / central university - there may be pockets of money to assist if you qualify.