I can't say I share that experience of a GWR Class 387 - was on a couple of them earlier this week and in one coach the air con was completely broken and in one of the others only half of the coach seemed to be being cooled!
The GWR ones are newer. The ones on GN are 2-3 years older (they started out on the traditional Thameslink route before moving across to GN last year).
Give them a couple more years and you'll be wishing they had opening windows for use when the air con fails.