Is it possible to limit the coupons only to certain plans via the UI?

Yes, when creating coupons via the Chargebee web interface, you can choose "Each specified item" in the "Apply on" field to do this.

When you create the coupons in Chargebee, you can select the plans that it should be applicable. Below is a screenshot for reference:-

To update applicable Plans, Addons & Charges follow the below steps

Open the coupon and click 'Change' next to the item.

When a coupon is created for Specific Plan, the discount amount is applied individually to each item specified. You can select the applicable item(s) from the following options for the discount.

1. Applicable Plans

a. All Plans

b. No Plans

c. Specific Plans

c. Based on selected criteria

If you have further question regarding this, please reach out to support@chargebee.com.