Hi
I think the most important part is agreement between indian debt company and UK Debt collection company if the agreement is in nature of professional service than there will be no requirement of GST and indian company can raise invoice without GST to UK Company
However if the agreement suggest nature of principal argent relationship categorically classified as "Intermediary Services " under GST Law than same will attract GST and need to raise invoice along with GST
So the crux is agreement draft between both the companies and scope of work defined.
We can surely help you to provide drafting of agreement or vetting of existing agreement to make sure there is no GST Element and you can save cost of 18% GST Presently levied including advisory on the invoices generated presently and nature of service defined