Normally a conservatory of less than 30 square metres in floor area does not require Buildings Regulations approval, however you are correct in assessing that if the old external door has been removed from the original house (where the conservatory access now is) then BR approval is required. This is because the extension is no longer regarded as separate, but has became an integrated part of the house.
BRs are there to protect you, the homeowner, not some random bureaucrat at the council. BRs ensure that the structure is built strong enough so it won't blow down, that damp won't rise up through the floor via the use of a DPM, and CRUCIALLY in the case of a conservatory that the house meets minimum standards for heating insulation.
This third one is the real killer for conservatories. Whilst they wonderful at taking a thermal gain from the sunlight in Spring and Autumn (providing a useful free addition heating source that can shift into the rest of the house with the connecting door open), in the Winter conservatories just soak up heat and are virtually impossible to keep warm. That is precisely why BRs say that the old external door mustn't be removed - in the winter you are going to find it really cold and you can't close it off. So the rest of the downstairs is going to lose out as well.