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Campanula, Agapanthus, hardy geranium, primrose, verbena bonariensis, vinca (check can be invasive), Echinacea are tough low maintenance...in the Belfast climate.
If you are more south of England, probably prairie perennials are more suitable as it's got so dry and hot kast year? Rudbeckia, Echinacea but check as the sluggies love some of those.
https://www.ballyrobertgardens.com/ is an RHS garden, practices Darwin gardening on clay here and they deliver, so maybe worth a look if your area climate and soil matches theirs
I'll be getting plants there.
Lawn is pretty essential with a small child. Other option would be fake grass but trying to avoid that. Being end of terrace it gets a decent amount of sun, we're hoping that the worst bare patches are due to the shade from the trees but we'll see this year.
Right hand wall is full sun and we're thinking some lavender and rosemary and a few other bits.
Back fence we're undecided. Would ideally like a fast growing, evergreen, thrive in full shade, flowering climber but can't seem to find anything that actually hits those criteria (hydrangea petiolaris was the original plan but isn't evergreen, hydrangea seemannii was the evergreen option but is reportedly slow growing).
Left hand fence some honeysuckle or something with low lying perennials (haven't worked out what).
Low maintenance and not dying all the time is a large part of the criteria.