avoid the cheap ukuleles, they are usually rubbish. On ones I have seen the tuners slip, the frets are badly placed so a mixture of open and fretted strings are badly discordant, and the strings are poor quality nylon line. They are just colourful toys.
You should be looking at paying around £60 for a decent quality instrument to start with (they go up to several hundred quid!).
Ohana are very good quality for the money at every price point, everyone I have met that has one has been very pleased with them. Kala is another brand with a good reputation; Lanikai (I think owned by Hohner) was one of the original good brands but seems to be uncommon now.
Try http://www.omegamusic.co.uk/collections/recorders-ukuleles-folk or http://www.southernukulelestore.co.uk/ .for specialists who offer a good service.