I have a 0-hour job for a year now with no contract.
You have a contract

I have a 0-hour job for a year now with no contract.
Isn't that only if you work full time? I am sure if you work part time you will get less days holiday.
paid holiday (bank holidays may be included in your holiday entitlement - check with your agency)
rest breaks and limits on working time
no unlawful deductions from wages
the National Minimum Wage
freedom from discrimination under equality legislation
protection under health and safety laws
details of job vacancies with your hirer
the same access to shared facilities and services at work as other comparable employees
https://www.gov.uk/agency-workers-your-rights/your-rights-as-a-temporary-agency-worker
I think honestly you're better off putting your effort into getting another job. If you're currently doing IT work for minimum wage then it shouldn't be too tough to find someone else willing to pay you the same, obviously dependent on location.
I can't see your work environment changing into a pleasant one any time soon, regardless of whether you get a written contract.
Seems like a decent move, an IT shop isn't really going to teach you anything. Go for it.
Also since you don't have a contract you don't have a notice period, which should appeal to employers.
6 months in and you've had 2 weeks with another week booked? Slacker.
Normally I'd think someone calling someone a slacker for having less than half their holiday entitlement in over half a year was being sarcastic, but this is GD so you can't tell.
GD as in General Discussion.
All you need to know is in the link below:-
https://www.gov.uk/employment-contracts-and-conditions/written-statement-of-employment-particulars
Failure to provide such a statement when requested by an employee is an automatic award at an employment tribunal - I've even caught out a big multinational (Hewlett Packard) with that one when one of my union members approached me and HR were slow in responding (as in not responding at all) - he was quite pleased with 4 extra weeks pay!
Cheers
My boss hasn't even told me my holiday entitlement