Ok, this is my area of expertise i think =) i work for a credit reference agency in the uk.
In answer to your question, a high balance on a card/loan will affect the lenders decision on whether you are accepted, if you are running high balances on overdraft/credit facilities it can make a lender think that you are credit dependent.
Best advice i can give overall is to check your report
Experian free 30 day trial
this will give you access to your full credit report for 30 days, free of charge (as long as you do not already and have not had an account before)
Please note, by signing up it will go on to a standard membership after the 30days is up for £14.99 per month, its non contractual so if you don't want it to, you can just call to cancel at any point during the trial.
Things that affect/may affect a lenders decision
- if you have applied for any form of credit in the last 12 months (advised no more than 2 applications in 6 months block)
- late or missed payments in the last 12 months
- Defaults/Bankruptcy defaults and bankruptcy information is on record for 6 years (even if you settle a default its still there for 6 years)
- Electoral role- a big one this, make sure you are registered at your current address, lenders like to know where you live.
Any more question, feel free to ask