let me get my crystal ball out..
This might all back-fire on Apple. Let me string a few 'ifs' together....
If Apple uses soft sims in it's phones, then the MNOs wlil stop subsidising them (why subsidise a product which might not use your network and you can't guarantee will bring you profit).
If the MNOs stop subsidising Apple phones, then customers will have to buy them at full price (£600+), probably direct from Apple.
If the MNO's stop selling Apple phones directly, then they won't have to sign up to the ridiculous Apple terms which force them to bulk pre-buy phones, therefore tying up less capital in boxes in warehouses.
If the customer has to fund the full cost of a phone, then they'll probably look around for cheaper offers and possibly turn to subsidised (and therefore cheaper) Android phones.
The market share for Apple will reduce immediately, plus people might not be so quick to upgrade next time around at full unsubsidised cost. Apple will have a smaller market share, and will therefore be less able to push the MNOs around.
The UK mobile phone market is different to the US one, so it's possible that they might not bring soft-sims here. It's interesting times for the mobile operators, and I can't see who the long-term winners and losers will be.