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I don't trust myself to not do something dumb when I'm riding supra-threshold intervals. I don't want to be doing the Hillingdon short circuit, checking for riders coming, dodging peds that are always walking randomly all over it, etc. I just like that I can hold power on the turbo without distraction. I guess it would be easier with practice and knowing your roads better. I tried a couple of times outside and it wasn't too bad but I do like the controlled indoor environment a lot. Next year, if I'm doing 12hr again, I'll definitely want more time outside on the TT bike.
hippy-
RP feels pretty safe for intervals, more so than Hillingdon.
The other place I've used is the towpath into Paddington. That is ok but you do have to pause in maybe every fifth one due to slower riders or peds. I've stayed off towpaths or any other shared use paths since COVID though. And towpaths further into London are way too busy at all times.
frank9755
Last year I did loads of their rides outside. Mostly Richmond Park, but I also did some on the roads around the Fifield TT course.
Basically intervals that are longer than about 5 minutes I hate doing inside.
In RP I mostly used the flattish bit from the Roehampton gate via Robin Hood gate and the path across the middle, with a turn of two as necessary, and an uphill finish from either end towards Richmond Gate.
That works fine as there's very little downhill I can't pedal and I can get the right gaps between efforts.
Fifield area is pretty good as flat and quiet (mostly), but I did screw up once or twice by forgetting where certain towns were (I once rode half way across slough with my brakes on trying not to lose an interval).
I've used hillingdon a bit too, more this year when RP was closed.
I've got a couple of loops round Ealing (inspiration your missus) where I hardly ever have to stop but they are a bit too hilly to work for intervals. But I'm sure other loops could be made round flatter bits of London with a bit of local knowledge. Chiswick by the river probably has potential.