I dunno much about Wales, but I knew a guy that grew bananas in OR, USA, prolly not much south of your latitude.

However, he grew where in the summer was almost all sunny days, and this is pretty important, I think, for bananas. Particularly if you are going to alter the wavelengths of light that is passed through the stained glass (sounds beautiful, BTW), you may want to research the particular light requirements of bananas.

Mayhap, you'll find it necessary to add lighting to allow the bananas to fruit.

I once grew a Monstera deliciosa (I call it a split-leaf philodendron, cuz mom did) in my living room. I acquired SAD (seasonal affective disorder) diagnosis, and to combat it I put a 1kw halide light in the LR. My houseplant flowered, and it was amazing. That's when I did some research to find out the plant was actually a food producing crop in the tropics, but the fruit is not commercially viable, for reasons not having to do with nutrition or taste.

Without high intensity light, they don't bother to flower. M. deliciosa can live almost centuries under the shade of the jungle canopy, without ever flowering, but when one of the giants falls, and opens up the sky, they fruit generously - and deliciously, hence the name.

Bananas, I'm thinking, might benefit from more intense light, or the entire frequency range of natural sunlight (which stained glass will change), and you might wanna think about that, in Wales.

Don't let that stop you from reaching your dreams! =)