UK Solicitor should be fine.
This form is for sponsors who have a co-signer (who is their common law partner) to meet necessary financial requirements to sponsor
other family members, not for people sponsoring common law partners.
Source:
http://www.cic.gc.ca/english/information/applications/guides/3900ETOC.aspWho must complete this form?
This form must be completed and signed by the sponsor and common-law partner, only if the latter is a co-signer on the application.
So, this form is not applicable in your case.