Lovely garden but yep, needs some colour. I planted a bed in my front garden of similar size. I went mainly for simple evergreen shrubs all I do is weeding and occasional trimming, very occasional dividing. The bare patches can be left for bedding plants or perennials.
Try euonymous,purple heuchara, hebes, variegated sage, berberis,
Have you thought about grasses and spikes plants? Phormium, uncinia, ophiopogon , carex and festuca.
A standard Salix flamingo would add height and looks great.
Hostas are fab but disappear in winter. pittosporum Tom Thumb is another favourite evergreen.
sorry about all the pics, my gardening is not great but I'm really enjoying taking and sharing photos and find it easier than descriptions , pictures speak a thousand words...
Dove's suggestions are so professional and will be beautiful - I think I'll dig a border out and replant...
I've just planted a choisya on verduns advice and can recommend it highly.
good luck I'm sure it will be lovely and watch out you'll soon have the gardening bug.
(an advert has sprung up in the middle of this -hope it doesn't show when I post this)