[Grut]

Weren’t the British planning to invade Norway themselves and Germany basically beat them to it (by only a few days)?

Yes. The Brit counter-invasion at Narvik used what forces they'd already assembled for their own Norwegian campaign. The British reason for invading was to deny the Germans Swedish iron (shipped via Norway) and to achieve complete domination of the North Sea. The Germans invaded to forestall the British move.

[PzLdr]

Yet, at Nurenburg, Raeder was not permitted to allude to the British plans as part of his defense to the count of “Plotting to Wage Agressive Warfare” regarding Norway.

[BroJoeK]

"The British reason for invading was to deny the Germans Swedish iron (shipped via Norway) and to achieve complete domination of the North Sea."

The Brits already had "domination of the North Sea."
So why would Hitler challenge them there?

NOT for the iron ore at Narvik!
As the article on page 7 above makes totally clear, the Germans didn't need Narvik -- not in April 1940, after Sweden's Baltic Sea ice had melted.
They could easily have received all the Swedish iron ore they wanted, with just a few million Reich Marks investment in new transportation equipment for the Sweden's route to its southeastern port of Lulea. Why invade if you can much more cheaply buy what you need?

So, mere Swedish iron was not the reason Hitler invaded Norway. Longer term strategic considerations certainly were the reasons. These included German bomber and submarine bases in Norway necessary to destroy allied Atlantic shipping.

Remember this: Hitler's decision to invade Norway came immediately after the Altmark incident, in mid-February. But the Brits did not decide to send their own forces until late March.

The result was, because of their better planning, the German invasion was highly successful, while the bumbling British, well, bumbled the job.