Basic compensation for injuries is based on the JSB Guidelines, previous caselaw and often the legal text Kemp & Kemp (Not Martin and Ross BTW).
JSB states that the compensation reward for amputation of one leg is £50,000 - £155,000, depending on where the amputation has taken place i.e. above / below knee. Any other damages would depend on future loss of earnings (i.e. a solicitor would get much more than a bin man) and other circumstances such as loss of promotion, disadvantage in the workplace etc.