I have to manually adjust the history file name.
You mean rename the t6 files? That's one way to do it, but it's not necessary at all. You can avoid it, and I'll tell you how.
Take your example, UK100. Broker A offers it under the symbol UK100_FOO, and broker B offers it under symbol UK100_BAR.
What you should do is maintain two different asset lists for each broker, but with matching asset names
Broker A asset list: Asset: "UK100", Symbol: "UK100_FOO"
Broker B asset list: Asset: "UK100", Symbol: "UK100_BAR"
Now both brokers use the same UK100.t6 files for UK100. No renaming required.