I would try and break things down into component parts. So:
Big Baby
Have you had a baby before? If so, how big? How big were you at birth, and any siblings? Did your mother have easy births?
Last time I looked, the NICE guidelines did not recommend doing anything differently for a baby that was suspected to be large - there is no way to get an accurate estimate of a baby's weight prior to birth (even scans can and have been up to 2lbs out either way!). Although some risks are higher for larger babies, those risks are still considered lower than the risk of intervention - especially when it is nigh on impossible to determine which babies are "large" in the first place!
Breech Baby
Have a look at www.spinningbabies.com for some ideas to try and get your LO to turn. If you can find an accupuncturist or chiropractor with the right experience, they do have some techniques that might help. For positional stuff I think the general idea is to try and get your pelvis higher than your shoulders (inversions), so that if they baby has started to engage gravity will help them out so they've more space to turn in. There's also all manner of fun (!) to be had with frozen peas, warm baths, music and torches - its a way to pass the time, if nothing else!
If your baby is breech then the current advice would be to recommend an External Cephalic Version (ECV - where they try and turn the baby while still in your womb). The national average success rate for the procedure is around 50%, but it is worth asking as some consultants have much better personal figures than that (possibly because they have the experience to recognise the babies that are very unlikely to turn before trying). If the ECV fails, or if you decline it, then they will almost certainly recommend a CS. This has been the NICE guidance for breech since a large trial was published in 2000 and found CS to be safer. Even though that trial has now been discredited, the recommendation still stands - and is now having the knock on effect that MWs are loosing their breech skills.
Breech birth isn't necessarily a foolish option, IF you can find HCPs with breech experience. If you want to know more, google Mary Cronk and/or try and get a copy of "Breech Birth" by Benna Waites and "Breech Birth: What are my options" by Jane Evans. I found both to be very informative and, IMO, more balanced in their presentation than the NHS HCPs I dealt with and I felt I was able to make a decision that was right for me based on robust data.
Back to back
IIRC a lot of the positional stuff suggested for helping a breech baby to turn is also useful for helping a back-to-back baby into a better position. At its most basic, adopt forward leaning postures wherever possible and avoid lounging on the sofa - tempting though it may be!
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Personally I'd probably worry about the breech issue first, as that is the one that will result in the recommendation for the most medical intervention - if you agree to a CS for breech then the baby's size and position becomes a lot less important.
I hope that gives you some ideas - fingers crossed the scan suggests all is within more normal parameters and the consultant is not required :)