Hi Santosh,
Although the question is specifically ask to Ms. Lakshita, I am also answering the question.
1. If I understood it correctly, you are only a commission agent for UK company and you receive your commission based on sales. In this case, you need to pay IGST. You need to raise invoice on UK company with GST.
2. There is a slight confusion here. However, in my view GST will be applicable on the expenses also.
Please feel free to call/ revert in case you need more clarity.
Thanks and regards
Abhishek Dugar
CA CS B.Com