what should i do for diagnostic radiography

Should i take a gap year and reapply to BCU (first choice uni which i got rejected with after the interview)

or should i got to coventry uni (2nd choice uni)

Why not do both? Start at Coventry in September and see how it goes. If by Christmas you're still wishing you were at BCU, then complete a new UCAS application and apply to there, whilst continuing to study at Coventry.
If BCU reject you again then you know it simply wasn't meant to be. However, if they make you an offer then you could choose to drop-out at Coventry and start at BCU.
This approach will attract an extra year of student loan debt and accommodation costs etc. - assuming you successfully start at BCU.
Is BCU worth that?

if i drop out half way through coventry in first year i still have to pay the full year of student debt?

I depends on when you drop-out: For tuition fee loans, according to https://www.gov.uk/student-finance-if-you-suspend-or-leave/print:
"Tuition Fee Loans
You’ll need to repay at least some of your Tuition Fee loan for the year that you suspend or leave your course.
You’ll need to pay back:
• 25% of the loan for the year if you suspend or leave in term 1
• 50% of the loan for the year if you suspend or leave in term 2
• all the loan for the year if you suspend or leave in term 3
This is repaid in the usual way once you start earning over the threshold amount."
For the maintenance loan (covering living expenses) the same document says:
"Maintenance Loans
Your student finance provider will reassess your Maintenance Loan based on the number of days you attended your course.
If any of your loan covers the period after you’ve left your course, this counts as an overpayment and you’ll need to repay it straight away.
The Student Loans Company will write and tell you how much you must repay. If you cannot repay the full amount, you can ask them to set up a repayment plan.
The rest of your Maintenance Loan is repaid in the usual way once you start earning over the threshold amount."
So the question you need to ask yourself is whether that cost (debt) is worth it to attend BCU. Obviously, if they don't offer you a place next year then you just stay at Coventry and incur no additional cost/debt.
One other thing to consider is that the student loan company with loan you funds to cover the normal length of your course plus one year. (The "plus one year" bit is to cover you having to retake a year, or something like that.) So if you drop-out of Coventry and start again at BCU, you'll have used-up that option, meaning that you wouldn't be funded for a retake year at BCU if that became necessary.

thank you so much this is a big help i’ll figure out what i want to do when i’m at cov uni