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.