Are some set to use UTC for the hwclock and some not?

Take a look at /etc/sysconfig/clock on your hosts, see if its consistent across your systems.

UTC=true would mean that the hwclock is set to UTC

You can run hwclock --utc but this always shows the time with the local time zone applied, but it should match your proper time if the london timezone file is correct. I wonder if some of your systems may not set to use UTC?