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Any employee can decide to retire at any time- the issue is whether there is any money in it. Your question appears to indicate, from the way it is worded, that there may be some sort of ill health early retirement provision in a company pension scheme.
 
If your employer does indeed provide such a scheme then the starting point for you has to be the terms set out in that scheme. With the pressures and costs on final salary schemes (it is unlikely that a stakeholder scheme will provide any ill health early retirement provisions, but it is always worth reading the small print), it is likely that such provision is based on you not being able to work at any job, not necessarily the one you were employed to do, ever again.

Check the scheme details. Incidentally, the decision in such cases normally sits with the pension fund trustees, not your employer.
 
If your question does not arise from a pension fund related issue but simply that you wish to retire I would urge you to consider two issues. Firstly, it is rare that someone who is disabled cannot work at all. You should consider with your employer what reasonable adjustments (this is a legal requirement for employers under the Disability Discrimination Act 1995) might be possible to allow you to carry on working even if it is in a modified role. Help in this area is free from Job Centres, Access to Work schemes and there are always occupational health consultancies available.
 
Secondly, if you haven’t already done so, I would check out the financial benefits you would receive as a disabled person- they are likely to be less than you would receive whilst working and indeed continuing working might well improve your quality of life.