This is a tough question indeed. Most single-word terms (such as around, about, approximately, or roughly) seem to mean "more or less"; most terms that express a small delta in one particular direction or the other (like a shade under/over, or a tad more than/less than) are not single words.
To express a little less than, you might try:
- The village is almost 30 km south of London.
- The village is nearly 30 km south of London.
- The village is scarcely 30 km south of London.
- The village is hardly 30 km south of London.
- The village is close to 30 km south of London.
- The village is not quite 30 km south of London.
- The village is just shy of 30 km south of London.
Of these, I like almost the best. Of course, the last three listed aren't using single words, but it's worth noting that the NOAD has a separate entry for the phrasal close to, defining it as "(of an amount) almost; very nearly."
Some might consider nearly to be rather ambiguous, in that it could mean a little bit less than (which is what we want), or it could mean approximately (which is what we are trying to avoid). However, the following usage note (from Collins) would lead me to believe that, in this context, nearly means not quite (or just under) 30 km:
If something is nearly a quantity, it is very close to that quantity but slightly less than it. He stared at me in silence for nearly twenty seconds.
A single word to express a little more than seems even more elusive; you might try:
- The village is barely 30 km south of London.
- The village is beyond 30 km south of London.
- The village is upwards of 30 km south of London.
Unlike close to, however, upwards of doesn't convey that sense of close proximity. If a village was close to 30 km away, that might mean a distance of 28 or 29 kilometers; if that village was upwards of 30 km away, that could mean a distance of 36 or 37 kilometers – at least in my mind. Same with beyond.
I think the best-fitting word might be barely, although, quite honestly, it took some deliberation for me to figure out whether barely 2 miles away would mean a little more than 2 miles away, or a little less than 2 miles away. After reading Matt's related question, and consulting the dictionary, I now believe it's a great word to convey what we want (i.e., just a little more than), but it seems like some of that subtle nuance might get lost in the communication. Note that Collins lists more than one meaning for barely, the first meaning would imply just a little more than, while the second (though informal) would imply just a little less than! I barely know what else to say!
All said, I still think the best way to convey these thoughts in English is by using phrases such as a little under or just barely over:
- The village is a little under 30 km south of London.
- The village is a tad farther than 30 km south of London.