I cant tell you where to get it in the UK, maybe it's known by another name? Try a fishmonger, it's a white fish. I think it's from the sole family. speakin of sole, Petrale Sole is very good too. Orange roughy used to be very inexpensive, but since it's popularity it of course, has gone thru the roof $$. I can get it a Whole Foods for $11-12 a pound, worth it though.
I found this:
Orange Roughy
Orange roughy (Hoplostethus atlanticus) is a deep water species, mainly taken by deep sea trawls at depths of 750 to 1200m.
Although the fish has a reddish body and bluish-tinged belly regions, it turns orange after catch (hence the name). The average size is 30-40cm but orange roughy can reach 45cm.
Orange roughy fishery
New Zealand pioneered the orange roughy fishery in the 1970’s and although catches have declined since then, we still remain one of the world’s largest suppliers. Catches are mainly from the Chatham Rise, the southern part of the South Island’s West Coast and some parts of the Challenger Plateau.
New Zealand's orange roughy fishery is managed by strict quotas, which allow only a set amount of orange roughy to be taken commercially each year.
Orange Roughy can live up to 149 years.