My personal opinions/local observations (non scientific):
Ivy has increased abit overall.
I think a mild winters have favoured ivy and also proabaly are less people removing it from trees. Also think some host ivy colonized trees might be weakened due to combination erratic climate, summer droughts etc and new tree diseases.
You see ivy higher up on crown/trunk unions on, healthy ash, larch birch, any tree sp the cast less dense shade than say others like sycamore beech etc, theese don't favour ivy as much. Taken to the extreme, rarely seen it flowering in a holly or yew crown. Also more ivy on on single isolated trees, or trees with narrower crowns that cover less area. So its all due to light lvls...
Ivy can cover the crowns of any tree if they are in poor health/decline...(as the light lvls increase due to a thinning crown)
An increase of the amount of ivy in the unmanaged hedgerow tree crowns(more light than in woodlands), esp on ash (adb) & hawthorn/blackthorn trees. . Alot of hawthorn tree crowns, do seem to get covered with ivy then the trees fall/blow over. Some trees are still alive and some are dead when they blow/fall over...
Unsure if death of theese hawthorn is due to the ivy or other reasons as if trees were 100% healthy doubt would ivy manage to cover and shade out the crowns?
Also has noticed an increase of ivy in flailed hedges. Flailing annually to the same "knuckle" seems to weakens the hedges vigour and thus favours ivy.
Cutting on say a 3 yr cycle instead would allow the hedge to shade out the ivy more imo.
Ivy doesn't as often claimed doesn't throttle/girdle a tree like honeysuckle can, or tree guards left on do, though maybe in rare circumstances this could happen?
Negative impacts on the "host" tree are only due to either:
Competition for light.
The increased wind sail effect.