The weight allowance is my main reason for using an Al BP.  There is
also a Kydex plastic BP available on divingniknaks.com for £75 which is
even lighter than Al.

Pack your dive gear in a golf bag. They normally allow more extra  (over
sport allowance) if you are a golfer :-))

I have both aluminum and stainless plates that I use for both fresh and salt
water diving.  So far, I've not traveled with either, but have made plans to
do so.  Here are my suggestions:
1. For travel, it's hard to beat the Transpac.  It is flexible, weighs less
than even the aluminum plate, and fits in to smaller spaces.  If the cost is
not prohibitive, that's what I'd travel with.  The only reason I don't have
one already is that, since my wife quit diving, I don't usually travel to
dive other than by car.
2. If you're not going to use a Transpac, an aluminum plate is the way to go
for obvious reasons.  It's lighter, a lot lighter.  Give some thought to how
you're going to carry weight.  Those of us that like plates, like them for
more than one reason.  I like my stainless plate because it takes weight off
my waist, all the weight in my warm water diving case, and has so much
better trim than anything else that there is no comparison.  You have to try
one, properly adjusted, to know.  I use Halcyon trim pockets.  They're
small, hold enough weight for my needs, the kit is almost as streamlined
with them, as without, and they work just as well with the hard weights so
common at dive destinations as with the soft weights I use at home.  YMMV.
The point is, knowing where you want the weight will help you decide how
best to locate it.
3. Whatever you try, try to get into the water with it before you go on
holiday.  The different buoyancy characteristics of fresh versus salt water
makes a large difference in choice and configuration of a wing, at least it
does for me.  My stainless plate is just right for my single tank, warm
water diving.  It is noticeably too heavy for diving in fresh water.  My 18
lb lift wing floats my equipment, including stainless backplate, just fine
in salt water.  It does not float it at all in fresh water.  I use a larger
wing when I dive my stainless plate in fresh water.  More often, I use my
aluminum plate when diving in fresh water.  It's too light for salt water.
I have to add the difference between the two plates by carrying more lead.
OK for travel because I don't take my own lead with me.  Not OK at home.
For my twinset, I use the aluminum plate.  My twins, without any hardware,
would be neutral when buoyant.  With hardware, they are always negative,
significantly so when full.  I have to carry ditchable weight to ensure I
can get back to the surface even with complete buoyancy failure and full
tanks, and I have to have more lift to support everything when I'm at my
most negative, so I do.

Hope something in the above helps.

Lee