As a customer you:-
Only need one estimate
Choose where your repairs are carried out
Entitled by law to hire a suitable replacement car (subject to conditions)
Are entitled to reimbursement of any financial loss
As per Vehicle Body Repairers Association.
"It's your car and your choice where you have it repaired. The Office of Fair Trading, The Association of British Insurers and The Financial Service Authority all agree that it's your right to choose who repairs your car."