Upvote:3
The visitor form has several options for marital status...
If you are still married, then select the option that indicates you are married. If you have a legal separation, select that one. You should then follow through in Part 4 where it asks "Does your spouse/partner currently live with you?"
...and answer it according to the current status at the time you apply. They don't care about separation unless you and your spouse are legally separated OR unmarried partners and the relationship has permanently broken down and that's not the case with you.
They will not refuse on the grounds that you are having a separation or have filed for divorce. In cases where a divorce has attracted lurid notoriety in the media, they STILL do not refuse on that ground alone.
You can always go to Part 9 and provide a comment, but based upon what you have written, it's not necessary.
You may be over thinking it. Answer the questions according to what the current status is on your application date.
Related post: Is it easy to enter the UK multiple times on the same Business Visitor Visa?