I'm sorry but I don't know how to delete/reinstall the dictionary, but in case you don't get an answer for that you should try "trashing" the InDesign preferences and/or re-installing a program (which is pretty easy if you have the CC version)
To uninstall go to the Creative Cloud application and choose Apps and then find InDesign. Hover over the name of the application and then click the gear icon. Choose Uninstall. Then after the app is uninstalled, click install. Doing this is probably easier and safer than trying to find/trash/replace the UK Dictionary. (I'd have trouble finding a clean, fresh UK Dictionary.)
The Hunspell dictionaries are not the same as the dictionaries that load with InDesign.
Ooops, right Sandy. I answered a bit too fast.
Can the OP give more information about System (Mac/windows?) and Indd version?
Still in case replacing preference don't help...
Answer : File was corrupt.
One file was giving me problems so i deleted preferences to no avail and reinstalled InDesign which had no effect either.
I pasted the text from the troublesome document into a new document and it worked. Somehow that file didn't like UK english. It was a new and simple document that i created the day before.
Yay! I had gotten the impression that it was all your documents.
Just in case others are wondering, here is the order of things you should try when things go wonky.
Try on a new file.
Save the file as an IDML file and then re-open.
Try with a new user or in the “Safe” mode for the operating system.
Trash the preferences.
Remove half the file, then the other half, other quarter, etc. until you find the page that could be the problem.
Remove any new fonts or extensions you may have installed
Some of these can be done in other orders, but they are the best set of “home remedies” before you panic.
Good advise. i'd normally be more logical about it but it happens so rarely these days i forgot