Thermostats do NOT alter pressure other than the fact they allow the water to get up to temperature which will increase the pressure, as it was ok for 13 miles not going to be this, maybe the rad was just shagged.
The viscous fan has no temp switch etc, it is fluid filled around the alloy centre, it normally rotates freely in raltion to the inner outer parts, but when a certain temp is reached the fluid goes more solid and trasmits drive from one part to the other to make the fan then turn at engine speed.
If there is according to the AA a lot of pressure in the rad, this would be an assumption on thier part as the pressure was now gone.
It's more likely to be a shagged rad, well it is now anyway or head gasket/s.
You have to buy a new rad so try it first, they don't suffer particularly from h/g problems.
tabetha