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Although the location in question is an island in the North Atlantic Ocean - so I guess the chance for some overcast during a couple of days is quite decent, even in June.
Best bet is to hope for thin clouds that diffuse but don't completely flatten the light or shooting into the shade as posted above. If those aren't possible then really try to keep your ISO low which allows you the most flexibility to pull shadows up in post processing. As always, keep an eye on highlights, especially the white feathers on Puffins or similar birds and make sure those don't blow out.
Nikon DSLRs support both Blinkies (Highlight warning) and histograms in image review. IOW, take a periodic test shot and review it with Highlights, Histogram or both enabled in the playback menu to make sure you're not clipping your highlights. You don't get a pre-capture histogram like you can in a mirrorless camera but it's easy and a good habit to occasionally review blinkies or histograms as light or subjects change to make sure you're not blowing out your images especially on a special shoot that's not easy to return to.
Much of the UK is north of the 49th parallel so the sun is not directly overhead though of course light can be harsh around midday, particularly if you get what you don't want – a clear blue sky.
Take several test shots and review with the highlights screen (Blinkies) as Dave suggested. Expose for the whites. Everything else can be brought up to proper exposure using the shadow slider in post processing. Good luck.
I think your post raises a good point in that everyone here has been commenting on the question of harsh lighting or blowing out highlights and sure these are risks but they're relatively easily managed with just a small amount of attention paid. I think the real danger for midday shooting will be the potential impact of distortion from heat differentials and atmospherics, something which I'd imagine might not be as much of an issue for trying to shoot a photo over open land in a place like Arizona where things might be pretty dry and uniform a lot of the time but might be a huge problem shooting over water in regions that are further north.
I assume you're headed to Machias Seal Island, and perhaps using Bold Coast to get there. There's a fair to middlin' chance you'll have overcast skies, so I'd be more concerned about whether the seas will be too rough to land than I would about light. While bright light isn't ideal, your subjects and setting should be enough to enable you to get some great shots. Enjoy!
I'm headed to Ireland. "Fortunately" the country is rather known from it's dynamic weather patterns - so it would be odd to not get any overcast at all.
