Can I swap a 700c onto a 26"/650b bike Can I put a 700c (28") wheel on a 26" or 650b fork, it looks like I'll have plenty of room but I would need to get something to move my v brakes as they wouldn't touch the rim
I assume you have a mountain bike, as you have a 26"/27.5" fork.

Answering the question in the title:

26", 27.5" and 29" mountain bike wheels all have approximately that diameter with a tire installed. Obviously the actual diameter of a particular wheel depends on the size of tire installed on it.

The rim sizes are 559, 584 and 622mm respectively, as defined by ISO standards. In road bike circles 584 and 622mm rims are also known as 650b and 700c - hangovers from a old French wheel size system.

So on a MTB, you need about 1.5" of extra clearance to fit a 29" wheel instead of a 26" wheel.   

Can you fit a 29" wheel on your 26"/27.5" fork? Not really.

Even if you have the clearance, adapters to move the rim brakes are not available (to my knowledge, maybe there is some speciality manufacturer out there).

In addition, bike frame and fork geometry is designed around a specific wheel diameter. If you change the wheel size, the steering geometry will be altered, usually with negative effects.