http://www.dti.gov.uk/er/work_time_r...7.htm#section7
This link might help. Under the Working Time directive, all employees (except some folks like police officers, prison staff etc) are entitled to four weeks paid leave per annum. This right is effective from your first day in a job. The definition of a week depends on how many days you work - e.g. for a three day week, your entitlement is 12 days.
Note that Bank Holidays are included in this total - you have no right to have a Bank Holiday off but you must get the day in lieu if you don't take it.
Note also that your employer has the right to set the rules on when you can take leave and how much notice you must give etc. The link above explains it quite well.
On the topic, I would also like to see T&C improve but I fear that legislative load like the above combined with the very low margins in GA training make things very difficult. I've yet to see an 'average' flight school that has rich owners (assuming they derive their income solely from the school) and poor staff.
I'm not as cynical as you are Esperanza (maybe I should be) - I don't think most employers are trying to take advantage - it's merely that the profit in GA training is not enough to pay us more.