650c wheels bicycle (for 5'3 and under)

Ah right that clears some things up, thanks.

I'm basically wanting to buy a road bike to convert to a single speed, I don't have much money and don't want to put a lot of money in to it because it will probably just get stolen in the city. I'm 5'3 with short legs!

The girls bike in the above picture looks a lot smaller than the guys. So the wheels are 650c but the bike will be a small adults bike? (like 50cm or something?)

Finding an old working road bike for under £50 seems difficult right now.

That pic is a bit misleading as that's a pretty large 700c bike -- probably something like a 58cm or 60cm frame -- making hers look that much smaller.

In the early days of triathlon, 650c wheels were popular on bigger bikes as they believed you could get lower in front and more aero. But with the advent of better frame designs and more knowledge (lower in front isn't necessarily more aero) that went away and now 650c is mostly for smaller frames. Probably hard to find a frame over 50cm these days that's 650c.