A number of employers take on people with criminal records of varying degrees. Unfortunately the offence for which you were convicted is not one which many (particularly retailers and the like) would choose to ignore.
The most important thing, as I'm sure you now realsie, is to be honest about your record. As you have found out to your cost, the truth very often comes out, and when it does the results are sometimes far worse than if you�d come clean in the first place.
The worst that could have happened if you�d disclosed your record was that you would not have got the job. True, you�ve had the benefit of two years� salary, but now you�ve lost the job, face possible prosecution for another offence, and certainly cannot use this employer for a reference in any future job applications.
Contrary to what Law Man says, your ex-employer has suffered a loss. He now has to go to the expense of hiring somebody to take your place (and will incur costs of advertising, interviews, etc.) and he may also have to hire temporary staff meantime. You can only hope that they will not be so vindictive as to encourage the police to pursue matter of deception
If you spent six months in prison I assume that you were sentenced to longer than that. The Rehabilitation of Offenders Act allows for convictions to be �spent� (i.e. non-disclosable) after various periods depending on the sentence imposed (not the time served). For prison sentences of more than six months and up to 30 months, this period is ten years. So, alas, you have a while to go before you no longer need to disclose your record.