This study is trying to find the right dose of a long-lasting medicine called GSK5784283 for people with asthma that remains uncontrolled even though they are using regular asthma treatments. GSK5784283 blocks the action of an inflammatory protein called TSLP that may be contributing to your asthma. The study will be conducted in two parts - Part A (dose finding phase) and Part B (extended dosing phase). Part A will assess the lung function, asthma control, participant safety and certain markers of asthma inflammation in the air you breath out and in your blood. Part B will assess the safety and long-term effects of the repeated or single doses of GSK5784283.