A lot of people, myself included, tend to ignore notams of 'high activity' events, or some high speed military flights, or survey flights, etc, on the simple grounds that these could happen anywhere
without being notamed.
One has to watch out all the time.
And it helps to fly high; most UK GA flies below 2000ft and flying at 3000+ reduces the traffic density by orders of magnitude. Above the cloud if possible because there sure as hell won't be any gliders up there (well not for long
) and it's easy to avoid marked gliding sites.