This sounds like a very stressful lead-up to what should have been a celebration for you, so you have my sympathies. And I also need to welcome you to Flyertalk donna2001. You best avoid seatguru, it's not accurate for many airlines, and BA in particular, thebasource is a better location, and by the sounds of it you are on a 747:

http://www.thebasource.com/seatmaps/...2j36w235y.html

Now, I'm not an expert on this cabin but as rapidex says if it was regarded as an emergency row then there is a very clear statement on BA.com seat selection menus listing eligibility, which includes mobility factors, and you'd need to OK this. Asking to sit in an emergency row with reduced mobility won't be allowed, but this should be OK. If this is a conventional travel agent then they should tell you what the upgrade price would be, alternatively you could ring up after T-24 (but ideally before you've checked in) and see if there is an upgrade price then, then try again at LHR when you arrive from NCL.

Good luck over the next few weeks, hopefully this will get easier with the passing of time.