Hello DDT,
It really depends on what roads you intend to take mate .. you can do the whole trip on motorways .. or enjoy some really nice old roads if you pull off and run parallel. Look at a good map and make the trip an adventure .. not a battle .. it should be fun .. if nothing else
Start as early as you can, crack of dawn is a good time to knock the first 100 miles off. Have the Bike all ready the night before, Bed Early. Up, Shower, Coffee and Roll it out.
Don't rush, set yourself a relaxed realistic time to cover the .. what will it be lets say 300/350 miles .. that really is easy enough in a day on a modern bike .. even in the wet .. it might be as long as ten hours with lots of stops .. but what's the rush .. if you start early you will arrive in daylight.
You Really do need to Stop and have a proper walk about and Drink a small Bottle of Water at least 4 times on the way down, don't eat much, no big meals or junk food .. maybe just a Mars Bar or some fruit
Stop well before you need fuel, well before means exactly that. That way you will stay calm throughout and not get yourself into panic mode about running dry on a strange road .. not knowing how far away the next pump is etc.
The trick is to maintain a steady average speed while riding within the legal limits .. not to get electronically 'pulled' for speeding or doing other silly things!
Don't let passing 'Birmingham' frighten you .. you will not be going anywhere near the city centre .. it's only roads.
Plan your route, just take it easy and Read the Signs. Put the Place Names in large letters on masking tape stuck on the tank top .. so you can confirm your route .. easily as you ride.
Above all enjoy yourself
Stay Safe Polar Bear (A Brummy in Spain)