2 years Employment Bond

My employer, a Big 4 consulting firm in London, offered me a job at 50,000 gbp with a 2 year bond. If I break the bond, I need to return the entire tier 2 general Cos, visa, legal fees incurred by the firm. Is bonded labour legal in UK? Could someone pls. advise what's the entire cost tier 2 general priority visa and cos ?

at least the visa fee, Ihs must be your responsibility to pay, not the employer. so this is ok if mutually agree. but the cost of COS allocation application? I don't think so, as your employer can allocate it to anyone, and this is the cost from the employer.