I think you may find that just doing testing might pall quickly and customers would probably not be impressed to have to pay you to test and then someone else to resolve any problems found during testing.
NICEIC registration is normally (inappropriately) requested by insurance companies and councils because they are ill informed, but if their internal polices require this then not being registered would be a limitation. However insurance companies will normally accept other schemes registrations after discussion.
You would need to ensure that the cost of registration for no service is worth it and since the schemes are there solely to provide a route for installation notification registration may be difficult if you have no installation work to demonstrate. I feel that the schemes may well be prepared to take your money if you were to discuss the situation with them.
Fro inspection and testing it is generally recommended that you get professional liability insurance ( I think most councils want 5m (instead of the usual 2m) Public Liability insurance as well for contractors working for them).
