My legal knowledge does not go that far but I am an employer and I will tell you what my understanding is.
(Unless you are on some kind of apprenticeship and I believe that employment terms are different in these cases) The fact that you had not been given a contract by your employers is neither here nor there, My understanding is that after a certain length of time, I think it is 3 months, your contract is implied and you are entitled to the same legal rights that any other worker is entitled to i.e. mininum pay rates, the legal minimum holidays, proper rest breaks etc.
I have never heard of a probation contract, you need to find out if these actually exist in law. Also I am not sure that your employers are acting legally by putting you on a probation period after you have been working for them for a year, it could be argued that if they had any problems with your performance they should have brought this up with you within that year, I am not sure that they would have a case if it ever got to tribunal. As far as putting in things such as 'we reserve the right to' in contracts does not necessarily make it legal, they still have to abide by the law in how they treat employees!
As far as the pay rise goes, as long as you are receiving the legal minimum wage I do not think you will have much recourse to what they have promised and what you actually get, apart from the fact that your trust in them as an employer dimishes.
My advice is to do what is right for you, have they put anything in writing about your agreement to pay them back if you leave within a certain timeframe? If so and you can afford to repay them without any problem, then I would definately look for another job. Not every employer is like this, there are some out there that care for their staff!
I really think you need to get further advice regarding your contract and working conditions.
Best of luck, hope I have been of some use.
Sue