The overriding factor has been stated by many on here. The Irish have (on the whole) the best owners, trainers, jockeys and Horses.
The way a novice chaser in the UK effectively begins it's career on the same mark as it's hurdle rating is one factor where I believe the Irish have an edge (as the Irish novice chasers start officially from scratch (kind of)). If you assume that the last hurdle rating for an improving hurdler is going to be it's highest more often than not. It is easy to see how the better novice chasers in the UK can end up on relatively high marks.
But as some others have said on here already, I think that the British Handicapper has slowly edged up the ratings somehow and the vast majority of British horses are a little bit inflated.
It's no coincidence that more and more novices (especially over hurdles) are winning the big handicaps these days. As the handicapper has not had it's chance to put it above 150 into the needs to be graded class levels.
I think one area I've seen in recent times is where a lower graded horse runs relatively well against a higher rated horse and end up getting penalised heavily, when a more realistic upgrade would seem fairer. An example of this would be Speredek going up 19lbs (135 to 154) for finishing 7l behind Un de Sceaux 162, but beating Kylemore Lough 154 by 14l, San Benedeto 157 40l and Brain Power 157 Fell.
That race is as good an example as any, as apart from Un De Sceaux, it's hard to argue any of the others are much above 150 based on overall form.