Special conditions that are imposed on a policy are not very common in the UK but when they do crop up they can cause considerable complications when applying for insurance in the future, since the fact that this has happened must be disclosed when applying for a quote. Unfortunately this usually means that many insurance companies simply don't offer a quotation at all, since they prefer to assume the worst rather than the best.
The most common special conditions refer to medical or physical issues. A driver with physical disabilities, for instance, may need a specially adapted car in order to be able to control it efficiently. A special condition of a policy may well be that only suitably adapted cars may be driven by this driver.
Alternatively the conditions may refer to the car which is being driven. It may be a particularly expensive car, or a kind which is targeted by thieves; Range Rovers in particular are very popular amongst highly professional thieves who ship the cars straight off to Africa in containers. Extra security measures for these cars maybe specified by the insurers.
Or, it may be that the area that the car is kept in overnight is particularly plagued by car theft or vandalism. An insurer may insist that it is kept in a locked garage overnight.
The important thing to remember is that if special terms have in fact been imposed on a policy, that this fact is disclosed to any future insurer. Since most insurance companies share data about their customers between themselves they would find out about this anyway, and if you failed to disclose it you would probably find either a substantial increase in premium on its way, or cancellation of the policy, which would be a very serious matter and would greatly affect your chances of getting insured in the future.
This is one of those cases where buying a policy online would probably not be the best way to proceed. It would be far better to either ring an insurance company and explain your circumstances to their customer services people; or get in touch with an experienced broker who can do all the work for you!