I am from the UK too and have bought my plants from B&Q or FOCUS. I have an umbrella plant, a weeping fig and a ficus (rubber tree) all were approx £10 each and fill the height of my 4ft cage.
They sell Ficus and Schefflera in various sizes in B&Q, I know there were some larger ones in my local B&Q the other day as I was looking for a small one for my crestie and they only had the large ones. The plants in Lidl are brilliant to wehn they have them in- I got a really tall Schefflera (Umbrella plant) for around £10. I don't seem to have much luck with Ficus plants - they have a tendency to drop their leaves for me, so I just use Schefflera now and Hibiscus when I can get them in the summer.