I was astonished by the number of mid-airs between military aircraft, especially through to the 70s and 80s.
Can't comment on civvie vs. Mil but mil on mil collisions sadly went with the territory and the time (Big air force compared with today, aircraft more basic, cold war tactics). Anyone who lived through that period will tell tales of the "F**** me that was close" moments or worse still tell tales of not realising you'd only just missed someone else until the debrief.
Why?
Firstly much more military aircraft around in the UK than there are now, all operating without any form of electronic collision avoidance. ( Look at the number of fast jet squadrons based in the UK now and compare with the number, say in 1980..)
Lots of formation flying ( not just the Reds, operationally as well, just about every sortie..shouldn't happen but there's always an increased risk of a clunk there)
Often as not operating (especially during training) in busy "open" airspace using "see and avoid" to hopefully prevent collision- e.g the airspace above Vale of York or Lincolnshire were general handling areas for several Basic Flying Training stations..on a clear day lots and lots and lots of Jet Provosts spinning/aerobating etc..
Down at low level almost everybody down at 250' agl, with high closing speeds, often with camouflaged aircraft, again only using Mk 1 eyeball for collision avoidance.