Sorry for the delay in replying.
That is not a bad sized pot and as it will be isloated from other planting you definitely want to go for something architectural as suggested earlier.
Fatsia japonica has been mentioned - easy to look after, has interesting (rather than beautiful) inflorescences of creamy white 'flowers' to compliment the green leaves when larger.
Aucuba Japonica also mentioned has spotted leaves and red berries.
Something with strappy leaves like a Phormium tenax or P. cookianum - these have very large flower spikes and should be happy for a quite a few years in a pot. There are plenty of cultivars about now that have different coloured variegation to the leaves.
Another 'strappy' choice would be astelia chathamica which will also thrive for some years in a pot and even take a modicum of neglect. If you can eventually get it to flower you will witness the most unusual orange berries. The underside of the leaves is a dramatic silver but this fades if out of the sun.
Some Yuccas won't mind the shade of a north wall and have impressive flower spikes. Yucca gloriosa variegata is a good choice for colour in the leaves although the plain green form will cope better woth the lower light levels. Love being in pots. Very spiky though.
Palms - A Chamaerops humilis or Trachycarpus fortunei will cope fine in shade and are as tough as old boots, the latter taking down to -18degrees C. The will both eventually flower producing yellow a inflorescense. Trachycarpus fortunei does not like winds and will look tatty if exposed. Trachycarpus wagnerianus has much stiffer leaves and will cope well with wind.
Mahonias are good choices - spiky customers and eventually growing quite tall but nice sprays of yellow flowers.
Something formal like a clipped box specimen (Buxus sempervirens) or a standard bay tree (Laurelus nobilis) will look great in an ornate pot but won't give you much colour other than green.
Some of the smaller bamboos will cope well with pots for years and give some height if that is required. Don;t let them dry out. A Fargesia species would be your best bet (avoid Fargesia nitida as this has been on flower for some years). I have three recent 5' divisions of Fargesia murielae 'simba' going cheap
here but I'm not sure where you are in the UK.
All of the above are evergreen for year round impact but you could go for something deciduous or herbaceous if you 're not bothered about winter. An ideal candidate would be Zantedeschia aethiopica, the arum lily.
I suggest you google the names for images and see what takes your fancy. The list is endless and we've only scratched the surface here.
Osteospermum Whirligig likes full sun
Please ask if you require further suggestions or advice.
Cheers
MarkEE
My Garden