It seems you have 3 options:
a. Bite the bullet and just keep the extra £6k in your AVC.
b. Try and get the £6k back. I would suggest contacting the trustees rather than the normal channels.
c. Investigate whether you can utilise any unused allowances from previous years: https://www.pensionsadvisoryservice.org ... ry-forward
I assume you’ll have to contact HMRC?
Paying into a pension is a great way of saving money when you factor in the benefits of tax relief and salary sacrifice(PSE). I don’t know why more people don’t do it.
I think the pensions centre only quotes you the maximum you can pay into your pension/AVC’s in total, because if they factored in PSE it may be seen as giving financial advice.