You need to be reasonably precise - as you have suggested, Birmingham or Manchester (and possibly Chester and Shrewsbury) are too large, and you need to be more specific. Naming a town is OK for a small town (Church Stretton, say) where the limits of the town don't represent a very large area. Depending on how close your route is to the target distance, you might be able to say "Kings Heath, Birmingham", or a named road junction, or a grid reference (post code better - they come up in Autoroute, which a number of the DIY organisers use). They will plug your control locations as nearly as they can into whatever mapping software they use to check the minimum distance.
With my local organiser, I can send an Autoroute file showing the control points to make life easier for him (he only told me that after I had tried to describe a remote location accurately on my last GPS DIY
). I assume from your comments you don't have Autoroute. You could get a similar result using GoogleMaps set to "walking", which you could then send to the organiser.
In my experience the organisers are very helpful.