So how will that work with the two platforms at T123? Will one be used
exclusively for London-bound trains that have gone round the loop, and the
other for bi-directional trains on a single track extension to T5?

That would prevent any T5 bound train entering the T123 platform until a
London bound train had cleared the extension and T123. That doesn't sound
very promising.

It would also be very confusing for passengers wanting to transfer to T5.

Or will it be one platform be for T5 only, and the other for both sets of
London-bound trains? (which would make more sense to the traveller, but be
logistically more difficult to implement, and result in a bizarre station
where twice as many trains went in one direction as t'other.)

That sounds more likely to me. It doesn't sound like it would be
logistically any more difficult. After all for the loop things would be
much the same and for the T5 link it just shifts the changeover from double
track to single track running to the other side of T123. It would also mean
a train could wait in T123 and head for T5 as soon as there the branch is
clear rather than waiting for T123 to be cleared too.

I don't think too many people would find the situation very bizarre.
Whenever I am at a waiting at a station there always seems to be twice as
many trains going the other way :-)