I live in a Grade II-listed mid-16th-century timber-framed cottage in Kent. I have dealt with various conservation officers, and they are reasonable, provided they understand that you have the property's best interest at heart.
It is useful to obtain the Listing to see when it was originally listed and if there are any denotable features. This will (as MikeG correctly said) define the extent of the listing, which is both inside and outside the property and up to the curtilage at the time of listing. Any adjoining fixed structure (garden wall, Neighbour's house, etc.) will also be listed. It may be that the property in question is listed because it is connected to the adjoining property that is listed. You will also be legally liable for any previous non-authorised alterations; however, this is rarely enforced, and you can purchase insurance for this, which is very cheap (see reference to the LPOC below).
BCOs and conservation officers have very different roles, and there are times when they conflict. When this happens, the balance of importance generally takes precedence.
I would actually open up a discussion with the local CO pre-purchase and discuss your plans - this will give you a feel what they are like and how easy they are to work with. The key is that you want to make the property habitable but ensure the character is maintained, but not to the detriment of the property, and so it is used and maintained accordingly. When dealing with COs, there are many ways to argue against their requirements (some legal and some practical) and some simple legal phrases to use that can often bring them back to reality.
One key point to remember is that you are preserving the "character" of the building, and this must always be remembered. If the alterations to the property can be undone without affecting the character, then this is useful to explain.
The Listed Property Owner's Club (LPOC) is useful.
Conservation Officers, like any profession, vary in their knowledge and abilities (most, contrary to various posts, are well qualified). It is often how you approach them and the issue that is the most important. For example, be clear that you have the property's interest at heart, explain what you want to do, an how and why and if it can be reverted to the way it was without detriment to the property, then you will be 90% there.
What is often forgotten is that repairs do not require LBC.
I would suspect that the CO will even allow double-glazed units if you explain that they will be timber construction with slimline DGU and in keeping with the neighbours, etc. One point to consider is that if you start replacing windows (and obtain LBC), this may open up issues for your neighbours if they don't have consent, which may lead to ongoing problems with your neighbours.