In reply to CurlyStevo:
its an interesting point this, and one that definitive answers seem hard to come by. its true snowdonia and the lakes are in the 'dark blue' parts of the met office rainfall maps showing highest average rainfall and most rainy days; but it doesnt show the peak values in those areas. it would be interesting to see whether another isohyet (i had to look it up...!) at 3000mm, or even 4000mm would still include the lakes/snowdonia, as the peak one is at 2,500mm. from its wiki page, ben nevis summit annual rainfall is 4,081 or 4,350mm; it had a weather observatory to generate that figure. snowdon's wiki page says 5,000mm- but the reference cited is a rather old textbook by the looks of it, and its not clear whether that is based on data from observations.
if it was as related to altitude as it seems, it would be expected that scotland's west coast would be wetter, as the mountains are that bit higher; and even if you compare like with like as you suggest, there is just more, and higher, high ground in scotland, so overall it would be reasonable to say its 'wetter'- though it would possibly depend on what each of us means by 'wetter'...
intuitively to me, the west of scotland would be wetter, if only because wales and the lakes are in the rain shadow of ireland, which may have taken some of the moisture out the air, whereas the westerlies hitting the west highlands have had 2500 miles of open ocean to get properly saturated.
but accurate data presumably exists only for places that have weather stations, which are absent through most of the areas of interest. seathwaite shows up as the wettest inhabited place in the UK, but i find it hard to believe that somewhere in knoydart or around glen shiel wouldnt top that.
all of them are virtually deserts though compared to puerto eden in chile, similar temperate maritime climate but mean 5,745 mm annually!
cheers
gregor