I have this "wall" of trees at the end of my garden that I want to keep for a bit of privacy but I was wondering if I could plant anything underneath them. Ideally any useful plants such as herbs or berries or anything that would survive in those not ideal conditions and would possibly bloom and make the side prettier. As non of the trees produce fruit or anything useful climbing plants like morning glory are fine too. Location is north east of England. Sun will be fairly limited.
Some kind of ivy is the obvious choice. Hedera hibernica is good ground cover, as are variegated forms of Hedera helix. But you do need to keep an eye on ivy as it can become horrendously invasive. Other spreading, drought-resistant possibilites are Vinca major 'Variegata', Hypericum calycinum and Euphorbia amygdaloides var. robbiae (just coming into flower where I am just north of London). Whatever you choose, dig nice big planting holes and ensure everything is kept well watered until the plants are established. If you're keen, you could try visiting any local RHS or National Trust gardens to see what grows where you are in similar conditions.
You can't plant directly underneath nor too close to the trees because of their roots, but you might be able to plant something 3 or 4 feet from the base of the trunks. If the area is shady, that rules out quite a lot of plants (all the ones you mention like sun), but something like Mahonia aquifolium, Skimmia varieties, and Brachyglottis 'Sunshine' should work there. These are all evergreen shrubs, and although Brachyglottis is a plant for sun, it will tolerate shade quite well - its grey leaves will lighten up the area, and it can be pruned as often as you like. Average height and spread 6 x 6 feet left unpruned. Average height and spread of Skimmia varieties is 3-4 feet, the Mahonia eventually makes about 5 feet high by about 6 or 7 wide. Note, although Skimmia does produce berries in winter, only female plants will do so.
If you wanted to plant some ground cover as well, hardy Geraniums such as Geranium Wargrave's Pink and Geranium macrorrhizum are good; for evergreen groundcover, Ajuga reptans varieties and Campanula muralis (now called C. portenschlagiana) will do well.
Nothing of much interest will grow under such shady conditions, and certainly not anything which will produce fruit. Herbs might be a possibility under certain circumstances.
Empty gaps under established trees is a frequent problem in established gardens. Once the trees reach a certain height they don't like as a general rule to sprout out lower down, and if they do then the growth is not of the sort you might want.
One alternative that avoids digging in possibly densely root-packed soil is to construct some kind of bench in that area to place potted plants. You could have tiered levels, like bleachers at sports venues for spectators, and then have a collection of herbs and other plants in varied interesting pots that can be rotated from a much sunnier area into the shady place for a few days and back out again.
Elderberries and (thornless) blackberries are often (and wrongly) considered weeds, but that is precisely why they are a good, fast-growing screen that can easily be adapted to the space requirements, and they do grow under trees (eventually). In my case, it took two years for both plants to start thriving under similar confined conditions.
If you cut them back massively in late autumn, they will grow even more the following year. Cutting them is actually rather essential if you want to avoid a messy look.
Last but not least, both of them have nice blossoms and have consumable fruits. Even if you do not pick them, the birds will thank you.
Your climate is probably a lot different than mine, but in my climate, in a similar situation, I've seen succulents with shallow roots do well. You can eat leaves of some species of Sempervivum, and they have herbal properties similar to Aloe vera.
Horseradish will probably grow just about anywhere (but digging it up might disturb your trees); they handle shade very well; horseradish plants are big and they look nice, though. Sunroots handle shade well, too, but again, digging them up would probably disturb your trees.
Corrales azafran safflower would probably work. It seems to grow in any conditions I've ever tried it in (heat, cold, shade, bright light).
Tarragon and mint might work.
Chives might work.
Deadnettle might work as an ornamental.
Tulips and similar bulbs would probably work.
Maybe some orchids.
I like Peter's suggestion of ivy.
Clovers might do well (the more shade-tolerant ones), and should add nitrogen to the soil (at least if they're innoculated). Ornamental clovers can be nice; I'm not sure if any of the ornamental clovers are shade-tolerant.
If you paint the fence white, you'll probably get more light reflecting on your plants.
Whatever you plant, I would be sure to take care of it. Don't neglect it just because it's out of the sun. I mean, make sure it has enough nutrients, water, and such, still.