Insurers can be nervous about people having standard car insurance policies but who commute to more than one place of work. This is because they have masses of statistics which they use when working out their premiums. They know what the statistical likelihood is of your car been damaged, vandalised, or stolen when it is parked at your home overnight, because they have figures for every single address in Britain.
In the same way, they need to know where the car is parked during the day; after all it will spend about a quarter of it's time there. If you have more than one place where you park it, the calculation can become more complicated and since they find it cheaper to sell their policies en masse, rather than examine each one individually, someone with non-standard requirements can sometimes be rejected by the computer system automatically.
This does not necessarily mean that the addresses that you have specified are particularly bad places to park; it is simply that, sometimes, their systems are not designed to calculate premiums on this basis.
It may well be of course that one of the postcodes that you have provided them with is a blacklisted one owing to high levels of fraud, accidents, damage or break-ins and so this is a different matter altogether. They would make this classification because they would know that there was a much possiblity than normal of your car being stolen or damaged at that location, so you may wish to change it for your own peace of mind.
An answer to the problem can usually be found by applying for business cover instead. Some people have found that this involves little or no increase in premium after all, but this will of course vary from one person and one insurer to another.
Business car insurance usually allows you to travel just about anywhere. It is vital however that you keep your insurers fully informed of the situation. If you apply for business use when you are in fact parking most of the time in one, risky area there is a very real possibility that any claims will be turned down and you may even be accused of fraud yourself.
As ever, it is vital that you are totally honest with insurance companies; not least of all because they know far more about us than we think they do.
A good specialist broker may be able to help you get a much better quotation.