Workplace pension

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If you won't receive the employer's contribution if you opt out of the scheme, then yes, you would be losing out - to the value of the employer's contribution.

You can pay into your personal pension concurrently if you wish.

If your employer will pay you their contribution even if you opt out (to do with as you like), then perhaps there isn't a great deal of difference outside of the charge caps on the workplace pension, investment choices, and not getting the tax relief at source.