Hi Tim welcome to GOTE its always good to see some new guys here you have a good start with the plants you have bought.
By the looks of your T.rex you may have to protect the growth point come winter as they can rot out when small but you would be very unfortunate to lose it completely.
The Musa basjoo should / will be ok in the ground the roots are good to go to -15 c I have tested that theory to -11c and they did come back just fine.
BTW Musa Basjoo that size in Wyevale at the moment are £52