Your kart is eligible for 125 open class as you say. Sadly there isn't much short circuit racing for 125 open anymore. Its mainly Superkarting UK which runs monthly meetings along side Darley Moor motorcycle club which is near ashbourne or the 125 long circuit championship.
Superkarting UK -
http://www.superkarting-uk.com/
British Superkart Website -
http://www.superkart.org.uk/
Also if your on Facebook we have an active discussion page
https://www.facebook.com/groups/534983186517728/
Also if you need any help on your Rotax engine, Phil Featherstone @ PFP Services is the guy to call.
http://www.pfpservices.co.uk/