Several things to note here.
1) Permitted development does not allow for changes to the front aspect of the house (I'd bet the local council put a section 4 amendment on the development) so building the garage would come under planning permission.
2) You have no right to light regarding the rear extension.
3) Even if they build over the pipe, they would still need to provide access if the pipe bursts / becomes damaged and in all likelihood they are responsible for any pipework under their property whether it serves their property or not.
4) Any build on your land without permission they can't do... near they can (even right up to).
I would:
Firstly approach your neighbours and mention that someone told you they were looking at extending and (being friendly) ask what they are thinking of doing. Going all guns blazing will just sour relationships with people you share a fence with so be nice.
Have your title deeds to hand so that if the conversation mentions they are going to build on your land, you can show them (and later the council) it does belong to you.
Check if your local council has an email subscription service for planning permissions near you. When a planning permission is logged with them within a certain radius of your home, it will send you an email meaning you can object immediately. They also usually provide free advice if you contact them to ask if you have questions (such as is building a garage over your driveway in your road permitted development or need planning permission).
You need to object on the grounds that these proposed works breach covenants on the property, that they affect the visual amenity of your home and the street in general (residential amenity), they encroach upon land that is yours, and the proposed development is over-bearing, out-of-scale or out of character in terms of its appearance compared with existing development in the vicinity (as it applies).
Note they throw out objections that mention loss of value to the home.
The key is to ask questions and get everything in writing from the council in case they do it anyway.