Also worth mentioning that all flypasts and displays, and often complete routes, certainly in the case of the red arrows, are publicised as NOTAMs (Notice To AirMen).
These are cryptic in text form unless you're used to having to comprehend them, but there's a site here that plots them on a map:
http://notaminfo.com/ukmapSometimes you'll see a complete route, but otherwise, you'll see points marked as "FLYPAST BBMF ACFT" with a reasonable indication of the time. If you find them all, you can get an idea of the likely route, bearing in mind that they often follow line features such as roads and railway lines, and that they will be keeping such features on their left while following them.
An idea of roughly where they'll be when combined with spotting them on flightradar24 should allow you to be prepared for their arrival.