Okay, firstly, the problem with this is that most European countries are in the EU and UN, so a war with a European country would be very improbable.
However, I would declare war on Romania, with the intent of taking over the country after winning the war. I chose Romania for their oil reserves primarily. I didn't have a particular reason for declaring war on any country, so I thought I'd go for one with oil.
'But other countries have oil too.' Yes, but in order to take over their country and therefore their oil, we'd have to win the war. Going to war with Germany probably wouldn't be practical as they have bigger and probably stronger armed forces than we do. So I went with a smaller country - Romania, whose army isn't nearly as big as ours nor is it as strong as ours.
Now, the attack. Well, based on the assumption that you couldn't whip a few tanks over Europe via air, they (and possibly aircraft) would sail around France, Spain and the south of Europe to the Aegean sea, arriving in Greece, travelling through Bulgaria and eventually into Romania.
However, it's pretty hard to disguise numerous aircraft carriers, and this would probably alert many countries' attention, so we'd also have the RAF on standby to attack if needs be.
I'm not entirely sure on the law of declaration of war, whether you have to declare war before mobilising any military, but if you don't have to declare war until you attack, then I'd do that.
Ideally, the attack would start east and west of Bucharest, in order to divide their army, and then using everything available to me (aircraft, tanks etc.) we'd move towards Bucharest.
Obviously the main hole in the plan is the interference of other countries, but like I say, inter-European war is unlikely, and this is all hypothetical.