I've fixed that now.
Region 2 (and Region 4) have got a lot more boxsets of linked stories available. Things like the "Kamelion Tales"
or "Ace Adventures"
sets are not consecutive stories, but they are episodes which are linked by a common character. Region 1 simply had them as individual releases instead of boxsets. Also there is the "Bred For War"
boxset which features every single classic series episode to contain Sontarans as the enemies. In practice it comprises various stories across a broad spectrum from 1973 to 1985, and three different incarnations of the Doctor. It might be a go if the OP is interested in seeing the history of the Sontarans (who have of course featured quite heavily in the new series in recent years).
I do think the two Season Eighteen sets are a very tightly-knit collection of episodes, and like I said they comprise almost
a complete season. "The Leisure Hive" and "Meglos" are the only two stories from that season that aren't featured in either of those sets, and both of those are more like stand-alone adventures anyway (although the ending of "Meglos" does lead into the E-Space Trilogy).