Originally Posted by
BA6501
Your first flight will have to be within one year of ticket issue date.
To get around this you could book e.g. MAN-LHR-XXX-LHR, fly MAN-LHR within one year and in September 2017 change LHR-XXX-LHR to September 2018.
This is quite a costly way to do it due to change fees even if it were to work - and as noted above I don't think it will since you are trying to shift the GUF booking beyond the allowed validity.