At Interactive Brokers (UK) Limited we offer a UK SIPP account, which differs from an employer run retirement pension scheme. Firstly you would need to establish a SIPP portfolio with a UK based SIPP administrator, and then have the value of your retirement fund transferred to them. They could then open an IBKR account as part of that portfolio, along with any number of other potential investments.
SIPP subaccounts are only available to UK resident individuals between the ages of 18 and 75.