The United Kingdom of Great Britain and Northern Ireland
Plenty of "R's. Definitely not Portugal.
Shares a border with France:
It's a water border, but still a border. And if you don't like that, consider the one in the Chunnel.
Has direct access to the Atlantic Ocean:
Cornwall, the Hebrides, Orkney, Shetland, and Rockall
Larger than Belgium and smaller than Poland
Belgium: 30,689 km2. UK: 244,820. Poland: 312,696.
South of Denmark:
The southernmost point of Denmark is 54°33´N near Gedser. The southernmost point of the UK are some rocks off the the Isles of Scilly at 49°51´, unless you count Jersey which is 49°8´
Finally the flag:
Clearly has red in it, and the lie is the lack of a cross.