http://www.southernukulelestore.co.uk is a good place to start looking.
I`d suggest concert size for an 8 yr old – small enough for the hands with little compromise on sound. Sopranos sound different levels of bad IMO.
Depending on budget. Something laminate for under £100, or something composite for over £100 .. it sounds like your child is quite talented and the instrument isnt likely to go to waste; so I`d go quality composite mindful of the knocks etc an 8 yr old will give it.
My eldest has a Magic Fluke which despite being bashed about a fair bit is still lovely sounding.