As previous posters have said the only regular seat to harness up to 25 kg here in the UK is the Britax
2-Way Elite.
However depending on the needs of the child and the vehicle in which the seats will be used, there are some other options but these do fall into the 'special needs seats' category. The
Britax Traveller Plus Si harnesses to 36kg and although technically a special needs seat, it can be used as an extended harnessing seat. Additionally the
Britax Junior Traveller harnesses to 25kg but again is a special needs seat. The
Britax Junior Traveller has been recently discontinued but there may be stock at some Britax dealers.
If special support is required then
Crelling Harnesses are worth looking into, although all models require use of the vehicle seat belt. The customer service at Crelling is superb and you can get "demo harnesses" for up to 2 weeks.
If your friend lives within feasible driving distance of Milton Keynes, I recommend getting in touch with the
In Car Safety Centre. The have a showroom with a wide range of seats, many of which cannot be tried or looked at elsewhere in the country.
I hope that helps.