May I second (or third etc) the Scottish suggestions. We normally go to Scoland for a fortnight (that's 2 weeks if "fortnight" isn't American) at the beginning of June. We always find the locals saying that the weather was better the week before, so May sounds good.
The longest day is June 21st, and in the north of Scotland you notice the long days. I have switched my car headlights on after 10 at night in the north, because of the time, not the light... Many hours to photograph in up there.
Fort William is one of the places we stay. Glen Nevis, which is nearby, has been described as "the loveliest glen in Scotland". I won't disagree. A word of warning though: the wet weather comes in from the west, so Fort William is one of the wettest places around, not helped by the fact that Ben Nevis is there.
The countryside changes a good deal depending on the underlying rocks. Aviemore is only two hours away by car, but the landscape in the Cairngorms is quite different.
Otherwise, you could do a lot worse that Yorkshire, which is a good base for touring (if you have a car), as well as being the most beautiful county in England (I'm a Yorkshireman). The Peak District is just down a bit, the Pennines are wonderfully scenic, the North York Moors have a wild appearance. And there's York itself.
If simply viewing the UK, rather than coming to photograph the scenery, a visit to Bath would be worth making, simply to see the Roman Baths. They have a good web site with photos, so you can check it out from the comfort of your own home before deciding. Fort William has a web cam, so you can always use that to see the rain <g>.