I don't know if they do day passes but -- may be just the ticket

http://athlete-lab.co.uk

Give Pat a shout at http://doendurance.com/
They have a studio full of kickrs which I'm pretty sure could accommodate you.

Or for a novelty you could train at altitude: https://www.altitudecentre.com/bookings/our-classes/

I live in London and you can definitely find legit "bikes" (not stationary gym bikes). Where are you staying? Bringing pedals/shoes.

Best option is likely Athlete's Lab (someone mentioned) - they have a ton of bikes and classes that are adjustable hooked up to Computrainers with massive monitors. Fun experience. Near St Paul's (the City - where a lot of the finance folks work). I took a similar class in Dubai and they upload the workout into your T Peaks and Strava afterwards.
http://athlete-lab.co.uk/

A friend is using a new studio called H2 in Victoria. I haven't been, but imagine it's a lot like Athlete's Lab.
http://www.h2victoria.co.uk/timetable


Wattbike - Various gyms will have Wattbikes (we have one at work) and it's basically an adjustable road bike and you control the power (or use one of their workouts). You should bring SPD cleated shoes though. I have a smart trainer at home which I do most of my workouts, but sometimes I use the office Wattbike and hook it up to my Trainerroad (you'll need an Ant+ adapter to record), works like an Erg trainer where you control the power. Great workout and pretty much the same thing as a Smart Trainer.
Locations here:
https://wattbike.com/kr/find-a-wattbike


I'm with you and usually find an indoor "proper" cycling option when I'm traveling to avoid traffic, etc...

2nd vote for Athlete Lab. Tell them I sent you... or not. It's all good. :)

Here's an overview of their setup, when I was there last month: https://www.youtube.com/watch?v=dlNqE6zIyY0

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Shane Miller - GPLama
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Thanks to everyone for the advice. I ended up going to H2 in Victoria (as it's close to my hotel). In case anyone ever need something similar, their setup is a cycling studio full of Stages indoor bikes which broadcast on ANT+ (and perhaps bluetooth). The room has air conditioning, but you should make sure you sit in its path (which I foolishly didn't do). There's the usual showers etc available, and everything was a very reasonable £10. They've also got a studio in Soho with (I believe) both Stages bikes and Wattbikes available.

Another venue I investigated (and would have gone to had it been a bit closer) was Cadence performance in Crystal Palace.

All the best,
Nir

For anybody that is interested I went to Cadence Performance last night for a Wattbike Class and it's a nice setup. £10 walk in fee, for London, wasn't too bad.

You def can do a one off. It's about 28 pounds I think, but I did a 90 minute workout for that. I've been there a few times while in London and got some friends that use it regularly. You can send them your fit co-ordinates so tthat the bike is setup to your liking when you arrive. Very good when you're time-crunched.