How many sheets of A4 paper would fill UK?

According to Wikipedia, the total UK area is about 243610km2. Since one square km has 1000x1000 = 1000000m2, this the area is about 243 610 000 000 m2.

Each sheet of A0 paper has an area of exactly one square meter. The area of A1 is exactly half that of A0, A2 half of A1, and so forth. 16 sheets of A4 paper thus cover one sheet of A0 paper, covering an area of 1m2.

Thus, you'd need 16 * 243 610 000 000 to cover the area of the UK with sheets of A4 paper.