You can retire anytime you want, you just have to figure out how to finance it, I retired at 55.

As I understand it the significance of 58 years age cut off is in terms of what happens when you leave your employer. In the past when you terminated your employment your pension pot was transferred to a foundation of your choice or the state one, where it sat until such time as you joined a new employer or reached your 60th birthday and during that time you received as
small interest payment or if you knew that it would remain there for some time you could move it to a limited selection of funds. In recent times there is a new option if you reach the age of 58 - your pension pot can remain in the former employers pension fund and continue to grow (or decline), I could be wrong on this, but I don't think you can continue to make contributions to it.

To the best of my knowledge there is no option where you can move your pension pot to some kind of pension fund after retirement, continue to make contributions to it and gain tax relief from doing so, but I could be wrong. I expect that there are financial products out there which would allow you to make monthly savings to it and would eventually be converted to an annuity, but not in the same way as you would do to a traditional pension. And it did not interest me in any case at the time.

One thing you need to be aware of is your obligation to continue to pay social taxes and the way it is calculated if you no longer have an income. It is calculated on asset valuation if you were the sole income earner and depending on how your set up financially that could prove to be expensive. This is something you'll need to investigate to ensure you don't end up paying too much and that you also qualify for a full state pension later.

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"There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living." - Nelson Mandela