Lymm Anglers use to have Ide in at least one of their waters, can't remember the name as not been a member for over 20 years. Might have been at Sandbach, Cheshire. They had 2 ponds but one went commercial and got dug much bigger by the landowner.
That reminds of a pond near me with course fish in it ide being one species as well as carp and rudd, there stocked but it's somewhere I'd overlooked in local waters, if there hard fighting I'd have a go at them.
I suspect they won't be comparable with a trout lb for lb?
If they are Orfe as Paul B mentions in a few posts above, a couple of lakes in the Newdigate area of Surrey have the Golden variety and they go to about 7lbs. I'm not sure they allow fly fishing though?
On this subject, there is ide, in Aboyne Loch in Aberdeenshire, a few miles west of Banchory. I have fished there for pike, and perch and people do fish for ide there, I know your in London and its a long way, but for others reading this it might be what they are looking for.
Also a bit further west is a place, called Tulloch fishery, really nice loch with rainbows, and there is some wild brown trout in there as the water feed is from a hill loch above Tulloch fishery.
A lot of commercial fisheries stock Ide, I had up to sixty odd pounds of them fishing shallow on the pole when I fished in matches.
Inevitably some escape in floods and do OK in rivers, I caught odd ones in matches on the lower reaches of the River Calder in West Yorkshire. Also caught a hefty one on Southfield Reservoir, which is connected to the Aire & Calder canal.
They can be quite predatory and snatch at moving prey, plus they like to feed in the upper layers, so probably fairly straightforward to fly fish for them.
Go on utube fly times fly fishing for ide they are fishing a river sand catching plenty and the chub one there on a small river/drain plenty of chub the only problem is it's in a foreign language but breaks up the day