I can't comment on waters in SW England.
There's five miles of fishing available on the R Wharfe in North Yorks at Bolton Abbey at ?20/day. I've had trout there averaging about 12 inches. I once had a brown trout at 3lb 120z, but this was exceptional. The water is stocked with brown trout, I don't know how often. Further upstream near the head waters I understand day tickets are even more expensive.
I should imagine if you fish small rivers containing wild trout plus a few stocked fish then the average size won't be very big.