The way I understand it, the invasion of Czechoslovakia made clear to the British policy establishment that there a war was going to be needed. According to Nevile Henderson the policy he as British ambassador to Berlin followed until then was that a) the Versailles treaty was unfair and created the conditions for the nazis b) if Germany was allowed to re-capture German areas (ie what was fair) by peaceful methods this would strengthen the peace faction of the nazi party and eventually lead to a normalisation (ie pre-WWI conditions) of the German society. My impression is also that for the nazi policy establishment the invasion of Czechoslovakia proved that Britain was not serious about risking war on teh continent.

After that the driving factor was the reenactment of WWI France and Britain had planned. Since defense had the advantage, invading Germany was ruled out (after some preliminary expeditions in Saarland). To lessen the costs for France this time the key was to open up more fronts and strangle Germanys access to raw materials. One such way was to get a Nordic front and stop the flow of iron to Germany.

Like occuping the minefields

During the Winter War the Norwegian authorities secretly broke with the country's own neutrality by sending the Finns a shipment of 12 Ehrhardt 7.5 cm Model 1901 artillery pieces and 12,000 shells, as well as allowing the British to use Norwegian territory to transfer aircraft and other weaponry to Finland.^[2]

This presented an opportunity to the Allies who, while genuinely sympathetic to Finland, also saw an opportunity to use the pretence of sending troop support to additionally occupy ore fields in Sweden and ports in Norway.^[11] The plan, promoted by the British General Edmund Ironside, included two divisions landing at Narvik, five battalions somewhere in Mid-Norway, and another two divisions at Trondheim. The French government pushed for action to be taken to confront the Germans away from France.^[12]

However:

The proposed Allied deployments never occurred, after protests from both Norway and Sweden, when the issue of transfers of troops through their territory was suggested. With the Moscow Peace Treaty on 12 March 1940, the Finland-related Allied plans were dropped. The abandonment of the planned landings put immense French pressure on Neville Chamberlain's British government, and eventually led to the Allied mining of the Norwegian coast on 8 April.^[12][13]

So try again

This was soon changed to a plan involving the mining of Norwegian waters to stop iron ore shipments from Narvik and provoke Germany into attacking Norway, where it could be defeated by the Royal Navy.^[19]

And this succeeded! Except the part about defeating Germany. And the part about reenacting the Western front from WWI.

So, in conclusion, Britain started war planning after March 1939, but their plans and action only makes sense in that they planned to fight the last war.

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