I rather suspected that was the case, so understood.

But many thanks for the shortcodes idea - that sounds promising and I'll have a think on how I can make that work for us in conjunction with UM's restrict access facility :)

I'm really sorry but there is no way to do this.

However, you can achieve that yourself by using the /%wpforo-profile-subscriptions%/ shortcode-urls, making some changes add a subscription in the menu section, then making some style changes via CSS code.

More info here: https://wpforo.com/community/faq/wpforo-shortcodes/#:~:text=wpForo%20Menu%20Shortcodes&text=Members%3A%20%2F%25wpforo%2Dmembers%25%2F,%2F%25wpforo%2Dprofile%2Dactivity%25%2F

I am also using Ultimate member, and our forum is members only (ie private unless logged in). So I've had to set UM profiles as the only ones that show on the site, not wpForo ones.

Is there any way members (or even just admin users of the site) can access this tool in such a setup? (I'm guessing the answer is no, but it would be really helpful if there was a way!)

I'd suggest you use the wpForo Subscription Manager tools. It can be found in My Profile > Subscriptions page

Does WPforo have the capability to display the "Subscribe for new topics" button on the main forum page? If a user clicks on this, they will then be subscribe to all the sub-forums and topics posted in the entire forum.

Appreciate if you could let me know if this can be done :) 

Thank you.