There really aren't just two poems that exemplify Romanticism in Britain. There are many. I can give you two that do it very well, though.
Wordsworth's "Tintern Abbey" demonstrates the romantic view of nature, its possible effects on human imagination and character, and Wordsworth's writing process. He demonstrates how he experiences nature, recollects it, then writes about it.
Shelley's "Ode to the West Wind" also reveals the romantic idea of nature working in human imagination--Shelley desires that the wind help him to do for poetry in the world what the wind does to the landscape.
Again, Romanticism is multi-faceted and is represented by a multitude of poems. These are just two that demonstrate romantic characteristics.