The option to opt out and select Custom postage instead will only be available for letter-sized items in the specific categories below that are priced £10 and under. The £10 limit applies to the item price only and excludes the Buyer Protection Fee.
You can find the category list here
Your only other options is if you make it seller pays for the postage. You'll still have to generate the postage label - but you can then apparently choose to use your own postage to post the item out and then get a refund on the unused label - though you have to request this.