Grammar desk
There hasn't be a groundswell of support, but then there hasn't really been any opposition, either. And no-one has told me that I can't do it. I plan to go ahead next week and start the page. Here's what I will do:
Create "Wikipedia/Grammar desk" with an introduction explaining the purpose of the page, similar to those on Wikipedia:Help desk and Wikipedia:Reference desk.
Post my "was a former" question to get the ball rolling.
Create links to the page from Wikipedia:Ask a question, Wikipedia:Manual of style, and anywhere else that seems logical.
Create "Wikipedia:Grammar desk/FAQs" that would start with my explanation of that/which and, with your permission, your explanation of its/it's.
Would you be willing to help me run it by helping to answer the questions? Do you have any suggestions?

I don't think there will be a lot of problems with UK spelling and grammar  there are a lot of UK editors. Sometime I'll explain Canadian spelling, which is a hybrid of UK and US spelling that gives us phrases like "labour organization" and "tire centre", which would not be spelled that way in either the UK or the US. Cheers,