The flight on 2/3rd is to Munich but booked via a work travel agency so BA will not allow me to upgrade the first leg, nor will the agency as they don't allow employees to cover costs themselves and work has a super strict travel policy. BA did say that I may be able to upgrade the return leg at the check in desk however.

So my plan is if I can upgrade the return leg for under £100 or so I will, and if not I'll ask for the 2 week extension and either book a return to Bordeaux which comes down to £50 using some Avios (thanks bisonrav) or the LCY-AMS with rewards points if that works out cheaper and/or a quicker journey (thanks Globaliser).

Thanks everyone for your fast and super helpful advice, very much appreciated!