Perhaps a “lost in translation” moment. Or the officer just didn’t know the rules. The DTV is completely new to most officers. Currently I’d speculate that none of them will know what the requirements will be to get a 180 day extension, so many may just not be aware of the need for 90 day reports.
Thai consulates in general will be closed on Thai national holidays (and any local government holidays). The 14th is a substitute holiday for the 13th which is for the passing of the late King and this year falls on a Sunday.
Not sure why you’d need an agent in Bangkok to help. You apply for it outside of Thailand. Use one of the nearby consulates that allow walk in or one of them that have switched to the evisa system.
A UK passport holder can enter Thailand with no visa for a stay of up to 60 days. You can even apply for a one off extension for a further 30 days at a local immigration office for 1,900 baht.
TM30 system is “unmanned”. It’s all automated. You enter the data and save it and it’s done.
If yours is saying “in progress” then you haven’t saved the data.
You can’t do a report using yesterday as an entry date [edited I meant “informed date”]. you do a report with today's date.
File again using today's date. Confirm the address at the top (you may have more than one address on your account). Enter the passport number, the nationality, the first name and gender (they are the only required fields). Then save the data. Once saved back out to the front screen and search on today's date. Your filing will show at the bottom.