The RAS have announced that NAM will be held at the University of Bath in July 2021, and a blog post on the University of Bath website goes into details on this.
MIST will be present at NAM at University of Bath to help celebrate the Royal Astronomical Society's bicentennial, which will still be celebrated at Bath.
No plans to replace the 2020 NAM with a virtual presence have yet been announced, beyond the following excerpt from the blog post above.
There is no doubt that physical meetings will always play a part in the way scientists network and collaborate, but we are also eager to explore digital conference platforms, as we believe remote conferencing will be an important step forward in giving our participants dependable access to talks and meetings. Such virtual meetings hold great promise in making meetings both more inclusive and more environmentally sustainable. Inclusivity and reducing our carbon footprint are priorities for both the RAS and for the University of Bath.