This is true, but its 4 years not 2.
However its is possible the conservatory might not have needed PP in the first place - folks use their Permitted Development Rights to build some extrensions and this have been what happened here. It is possible to apply for a Certificate of Lawful Use from the council if PP should have been sought but wasn't and more than 4 yrs have expired. This is not your problem and the vendor must sort this out (to the satisfaction of your solicitor) or just don't buy the house. If you do buy, you will just acquire the problem.
If the Conservatory is bigger than 30 square metres in externally-measured area, the structure should also have had Building Regulations approval - irrespective of whether it needed PP. BR approval confirms the structure is built to minimum quality standards - conservatories less than 30 sqm are regarded a temporary, non-habitable structures, and hence never need BR approval. Above 30sqm they must have it.