I sent them an email asking to close my account and got the below response:
Unfortunately we are unable to close an account if there is outstanding bad debt on the account, and currently there is -£226.58 left outstanding on your account showing as bad debt owed to you.
As we have a dedicated Collections and Recoveries team that will be working to recover funds that were lost when these loans defaulted, you may receive further recovery payments into your available funds, and therefore we need an account open for you in order to allocate these to you.
Should you not wish to pursue this option you are also able to close your account by donating the residual money in the account to charity. You are able to assign the rights of your account and the non-transferable loan parts/losses to Funding Circle. Any money left in the account as a result of repayments or recoveries for the remaining loan parts will not be retained by Funding Circle, but donated to our chosen charitable organisation.
Our chosen charity from 2019 is Whizz-Kidz (Registered Charity: No 802872), a charity that works closely with transforming the lives of disabled children across the UK.
Should you wish to pursue this option of closing your account, please find attached a letter of consent that will need to be signed and returned to us, either as a scanned attachment by email or a paper copy posted to our office address. Once received, your Funding Circle account will be closed.
I sent off the form and account was closed shortly after. (Though I doubt the charity got any money out of it!)