How importatant was the Commonwealth (British Empire) heavyweight title?

If you're asking about the 1880s and 1890s, then it's simply The British Empire title, not "Commonwealth".

I'm guessing it was quite important. Didn't Peter Jackson hold it ?

There's actually some historical controversy, and always was, over whether John L. Sullivan and Jim Corbett had a rightful claim to be "world champion" under Queensbury rules. Some say the lineage they held only entitled them to be American champion of Queensbury rules.
Events in England, and the British Empire, were significant at the time.

 

I thought it was the Lonsdale Belt that's important in Commonwealth boxing. There you earn the title "outright" after 3 defenses of the British title in any BBB of C recognized weight class. Or does that only apply to Great Britain and not the Commonwealth?

Edit: Oops, you said 1890s. The belt wasn't brought out till 1909.

 

It was crucial.

At the earlier end of that period it was virtualy impossible to establish lineage without it.

The world title as we know it today is basicaly the ******* offspring of the British and American titles.