Well, quite.
So where was it flying to/from?
I don't think they have the range to get here from Mildenhall.
Of course they have the range to reach Lewis and return to Mildenhall (the return flight is about 800 nautical miles), but that wouldn't give them a lot of flex. However, Mildenhall also is home to the MC-130Js of the 67th Special Operations Squadron. One of their roles is to refuel the Ospreys in the air, and that's a skill that needs regular practice (I used to teach it, both as dispenser and receiver), so I'd be surprised if that wasn't part of the trip. In UK, air-to-air refuelling is normally practised in one of several oversea AAR areas (it's not normally allowed over land at the altitudes the V-22 can do it), and the role of both aircraft also requires them to be able to fly at low-level over land. The west of Scotland provides the largest and best low-flying training area in UK, so to combine refuelling practice in AARA 14 (see
here) with some low flying over the Highlands and Islands makes for a good day's work.
If AAR isn't part of the trip, they have the option of stopping for lunch and a top up at Lossiemouth or Leuchars (now Army, but still available to aircraft) before or after the low-flying bit before returning home. Prestwick also has frequent USAF visitors, though I don't believe there's any permanent military support any more. And of course Stornoway is also very familiar with military users having been part RAF until 20 or so years ago, and still used for exercises from time to time.