Originally Posted by Sir_Loin_of_Beef
Most places around here use Pacific red snapper, which is actually rock cod, and will upgrade you to halibut for an additional cost.
CF, since you are in Acton, my guess also would be that it was Rock Cod (Red Snapper) and a wonderful choice for fish and chips. I actually prefer it to Halibut for this dish, although I adore grilled Halibut. We have a wonderful harbor restaurant here in Ventura called "Andrea's" and they have the best fish and chips made with Rock Cod I've ever eaten anywhere, including the United Kingdom.