Generally, insurers try to steer you into using their repairers, I would not be complaining as it is better that they are providing you with the correct entitlements. You can, however, ask them to arrange repairs through their network of repairers dependent on who you are insured with. It may be they do not have their own network, if not, contact the nbra.org.uk/ and ask for their assistance in recommending one for you.