I'd say #1 is Thomas Malory + Le Morte D'Arthur. It's romance, epic in scale,
Tolkien would've read it, it's a foundational text in many ways for plenty of other literature. I'd say easily as important as
The Lord of the Rings.
An Oxford Uni Press book which is fun for fans of romance is Reynard the Fox by Anne Louise Avery.
Both are more continental in tone and inspiration than LR, which is more Northern. You could continue with stories of Charlemagne.
From native Britain:
3 William Morris
books: The Wood Beyond the World (heavy on landscape), The House of the Wolfings (set in
Mirkwood) and The Story of the Glittering Plain (his others probably too I've only got those 3) + some George MacDonald and it's all on JRRT's trail.
TBH I find Thomas Hardy has some affinity in the nature aspects and mode of writing