If the external appearance of the structure from the outside has not been altered, it is very likely that Planning Consent is required. However virtually all of the changes you mention required Building Regulations approval and without that, you have very little idea to what std the work has been done. Knocking two rooms into one is particularly suspect.
Unless you know whjat to look for, I'd steer clear - this situation is likely to be worse than an unmodified house - at least one knows it has been built to a standard and not tinkered with since.