A full explanation of creating rules can be found here
If find a shipment has an outcome you didn't expect, check the resulting carrier and service and any rules that output that carrier and service.
Our shipping rules engine works with a cascade effect, starting by checking the rule with the most critera.
You may have 2 rules.
Rule 1. Any shipments that are over 10kgs and going to the UK are to be sent as Royal Mail 1st Class Mail
Rule 2. Any shipments that are going to the UK are sent as DPD Parcel Next Day
A shipment comes in, that weighs 15kgs and is addressed to the UK.
The outcome will always be Rule 1. As it has the most critera to match.