Other than one occasion, I can only say that ATC have never been anything other than professional and helpful, no matter how busy they've been.
On that one occasion, I was requested by Birmingham Zone (who were excellent) to contact Coventry as a courtesy call, because I would pass close to (but outside) their ATZ. Coventry didn't want to talk to me - on the third call they simply replied "G-XXXX stop calling us and standby". And so I waited, and waited. After ten minutes I was well away from Coventry and so returned to Birmingham Zone, who were once again excellent. I wouldn't have minded but Coventry didn't seem that busy (on the radio anyway) and would have had plenty of time to talk to me. In hindsight I should have just made a position call and then stated I was returning to Birmingham frequency.
Every other time I've dealt with ATC, whether for LARS or zone transits, they've been superb and I really appreciate their help. I always try to sign off with "thanks for the service" because a minor courtesy (not taking too much airtime) costs nothing (rare in aviation!) and shows my appreciation.