I wouldn't be so dismissive of your GP either. I've been referred to NHS physios for a number of running related injuries over the years, particularly if you stress the possible impact on your overall mental well-being of not being able to run!.

Although it often takes time to get the appointment to see them (chasing for cancellations helps...) my experience is that they know their stuff and are much keener to get you sorted and on your way than private physios, who are often after multiple repeat visits!

Many knee injuries can be eased by doing lots of squats, but it helps to have that diagnosed properly of course!