You have a very confusing list of options there, ranging from a childs bike, the alimay link, to a 16" to a 20", being as you are in London, you have a vast choice of good bike shops to go and have a look at, from chains like Evans and Cycle Surgery, big box like Halfords & Decathalon and independents and local chains, find out the size you need, then look again.

Although your budget is limited for a good MTB, you have hit the right time to look, being as 2014 models are being cleared for the 2015 season, if you don't mind a last years bike, try places like Triton Cycles or Pauls Cycles, pushing you budget to £600 could help a lot.