1000 miles in 24 hours is 41.7 mph average. And that's not including stops for petrol, food, toilet, etc.
That's pretty fast for an average, and as Nick hints, Welsh roads aren't very straight especially around the edges. You're going to need some open A roads.
Maybe add a few runs down some of the bigger straighter roads in the middle, and maybe in the Marches, A49 etc? They are very nice roads, and reasonably empty compared to a lot of England.