It seems like for some reason the broker is not allowing that much UK100 history to be made available from the feed. One workaround is to use the PRELOAD flag which would use your .bar history and then only request the most recent live feed data from the broker. Here is the man page for PRELOAD

Another suggestion, if possible, is to reduce your LookBack by about 40%. Then there would be enough UK100 available without using PRELOAD