I'm thinking do not take out more than a third of the leggy canes that far down. It's been awhile since I had a hybrid perpetual, but I treated it like a hybrid tea and cut it in the spring. I'm thinking you wouldn't want to stress it too much by taking too many canes in this warm weather. I have a large climbing rose, and it's said to cut it back when it finishes blooming. Problem is, it never quits blooming, here it often blooms until Christmas! So in September, I cut back one third of the canes, and totally remove the oldest ones. This works. The one third rule is pretty much across the board for all shrubs, whether trimming for shape or revitalizing.
You mentioned the spring flush, well my hybrid perpetual bloomed throughout the summer. You might just want to wait until September or so when they slow down to do any hard pruning. All my roses have blackspot and are losing their bottom leaves despite my vigorous spraying. I will prune a little harder in September. Doesn't seem to hurt them as we have no really cold weather until later. Now last year we hard a very early hard frost, and froze blooms and buds on the shrubs. I just left them until spring, in early February, the normal time here to cut back roses. Early hard frosts aren't the norm here.