what to plant now

As I said in my post, they're ok for a few years in a pot, but your criteria is limited to a handful of things.
Alpine strawberries are small, plants getting to about 4 inches high with fruits the size of malteser.
Go with that but you lose the wow factor.
Soft fruit bushes normally means you give up the evergreen wish and the wow factor.
Aloe striatula or agave montana succulents to look at. Evergreen, hardy and quite a bit of wow, but no fruit.
Sempervivum in different colours for evergreen and a tiny bit of wow.
Small leaved hebes will give a compact, evergreen, attractive look with limited wow appeal, but whatever you go for, it'll be a compromise

 

I think if you cut out the fact that it needs to produce edible fruit you will have much more chance of coming up with something suitable that will also look good.

 

If you drop the edible requirement and put holes in the base of the pot you could try Taxus fastigiata aurea, will need trimming from time to time, but will probably cope with the conditions unless you live in a really cold area of the UK. Such as areas labelled 6 & 7 on this map Hardiness Zones in the United Kingdom

Alpine strawberries height no more than 10 - 15 cm.
If you are in SE England then Opuntia may well do quite well in the rain shadow of the property.
Arbutus unedo is so named for a reason.

 

Aren't there currently fairly dwarf forms of Arbutus? I've been struggling to grow rubra here and the best specimen is only 1.5m after 10 years.

 

I'm not sure a 30cm deep pot is a great environment to keep a lot of plants long term but the Japanese do a good job of something similar with the art of Bonsai that keeps a variety of trees at an unnaturally small size with particular techniques to how they are cared for to give them the shape of the mature trees in miniature. Below is a picture of a Bonsai Juniper communis which would be my suggestion of what you could plant in it to meet the features you mention you would like but the traditional way these are grown uses a shallow pot. If the place you have in mind is south facing and some way sheltered some other options could be myrtle or Feijoa but I would not expect much of a fruit yield in a 30cm deep pot. In my own garden I get plenty of Myrtle fruit but only flowers on the Feijoa.



Happy gardening!

 

Need a lot of drainage for cacti.

 

Opuntias especially are not something you grow unless you really like them, the spines on most are bad enough but they also have glochids, tiny hair like spines that are difficult to get out.

 

to be honest im not keen on any of these suggestions.
I might buy alpine strawberry for the back

Can anyone think of a plant that teas can be made from that will be green throughout the year and can be grown in a pot outdoors

 

You'd need a bigger pot for Camellia sinensis.
However Rosemary or Sage would work, both would require good drainage and preferably a sunny spot, not sure how they would cope with cold winds, if you suffer from those.
Neither are green, more grey green.

 

If the spot is sunny, Rosemary would fit the bill well. It's very attractive evergreen shrub with lovely blue flowers in late spring.

Rosemary is well known kitchen herb but it also has a (very) long history as a medicinal plant and plenty of beliefs and folklore too if you are interested.

Other herbs would be fine too, most need sunshine but are otherwise draught tolerant and can be used for teas or infusions in addition to flavouring food. Thyme is evergreen and Marjoram and Mint semi- evergreens. Lemon Balm is nice too.

Herbs will not look spectacular in a small container though- they are modest plants. (Digressing here... If you are interested in growing your own herbs, nothing is easier, get a 50cm container and group in your favourites. Supermarket herbs do well all summer and many are perennial. )