dag
Crossing in to Gib is no fun as the Spanish govt makes it difficult even though the majority of the 'crossers' are Spanish.
That's not the case in my past 8 years' experience dag. Getting in is always very easy: either by car/foot from Spain, or by air from wherever - wave the UK passport and glide through. Getting out can be more difficult, depending on traffic/ customs-of-the-day/political situation. But mostly, absolutely no problem, and very swift and hassle-free (but when the Spaniards are feisty, it can take HOURS - very infrequent though, and not the norm).
TIP. If you're going from Spain by car, leave it in the undergound car park on the Spanish side, walk 200 yards across the border, and get a cab to wherever you wish to be (£4 at most). Vice Verca. Simple. And Customs is always a breeze when on foot.