That could be lice eggs. Lice are different from mites. Get some poultry dust (permethrin) or whatever lice/mite treatment is approved in the UK - liquid or dust.
Treat the birds and the coop (and toss bedding/nesting materials) with the insecticide. Repeat in 7 days and again in 14 days.
For the nits (the eggs), after all the bugs are dead, I have read on BYC that unrefined coconut oil will dissolve the nits over a period of maybe a week. You will have to soften the oil by warming it. But this is a cosmetic issue. Some trim the feathers. This is not the treatment for the bugs, though. The treatment is the insecticide dust, or (diluted as the label instructs) liquid sprayed onto the bird.
Make sure you treat under the wings, vent area, and everywhere but the face.
I have only dealt with mites, not lice.