I'd guess yes unless you had an overzealous dog warden like they have in Exeter. Many citizens now believe they are on commission the amount of fines they hand out.
A harness might be accepted as a suitable substitute for a collar by a dog warden or police officer but a court would be obliged to refer to the wording of the Control of Dogs Order 1992. This specifies that a collar (and tag) must be worn, and makes no provision for any alternative: http://www.opsi.gov.uk/SI/si1992/Uksi_19920901 _en_1.htm