What To Plant Under Holly Bush On Small Slope?

We have cut back this holly tree/hege which began to overtake the lawn at the rear of the garden. This, together with the ivy on the far right. There's not much more we can do with these two hedges seeing as they're on our neighbours plot (we'd love to eradicate the ivy!). If you have any further advice on our pruning, we are all ears .

This bank is West facing and gets the sun from around 2pm until sun set. We don't want the holly to overtake the bank again as it makes mowing the lawn a pain. However, we also need to do something with the soil bank. Not sure whether there's a need for a retaining wall, or just some ground covering plants? But what would grow well under this established holly?

All ears and all suggestions are very much welcome!