What type of 'ab-initio' and/or 'instrument' instruction? If you mean instructing towards a JAR-FCL PPL(A), then no - you'd need to hold a JAR-FCL FI Rating. To transfer a FI(A) Rating from a non-JAA state licence, you will need to hold a UK or JAR-FCL aeroplane licence which includes a valid SEP(Land) Class Rating or single pilot, Single Engine Type Rating, to meet the knowledge requirements for the grant of a JAR-FCL CPL(A), to have at least 200 hours of flight time on aeroplnes including 150 hrs P1C if a PPL(A) holder or 100 hrs as P1C if a CPL/ATPL(A) holder. Then you have to have also flown various hours requirements (total, instrument, cross-country) before you fly the pre-course entry flight test. If you pass that, you may then start the FI course (15 hours of flight training and 30 hours of ground training) - then you take the FI Skill Test with a suitably qualified Flight Instructor Examiner (Aeroplanes)......
However, if you mean can you instruct towards a FAA licence in G-reg ac then sorry, I don't know.