So the Wii U's european PAL library has a considerable number of games that have a catalog/item number ending with UKV. Is this the official way to distinguish the UK version of the game(s)?
Yes, it is.
I've seen that some site editors edit game titles to include [UK] in the title, yet some games that have the UKV item number do not have the [UK] addition. Why is is?
It is because we don't have enough time to edit all those games in our database. If you want you can edit it and add [UK] to the title.
These UKV games are also sold in some European countries outside the UK.
Yes, they are, but [UK] stands for the origin country of release, not where you can buy it.
Is the UKV release of a given game considered the base/standard European PAL release?
No, it is not. UKV is for UK, FAH is for France and Netherlands, FRG is for Germany, ESP for Spain, ITA for Italy etc. Some of the Nintendo relases has UXP or EUR which means that they are "base" releases for whole Europe (multi language on the cover).
Hope that I explained it a little