If your chargeable event gain is over £10000.00, then you meet the criteria for completing a self assessment tax return.
Chargeable event gains, should be declared on SA101 page Ai1, boxes 4 and 5 please see Self Assessment: additional information (SA101)
Self assessment will calculate the top slicing relief for you.
If you are completing your tax return online, when tailoring your return, select yes to "Did you receive any other UK income, for example, employment lump sums, share schemes, life insurance gains?" onpaqge 2 of 3.