There are no theoretical knowledge credits. There is an exception to this if you have an ICAO licence in current flying practice and have a minimum of 50 hours Pilot in Command in Instrument Flight Rules.
You must hold a current and valid: ICAO licence with a validating medical & Instrument rating. The theoretical examination is assessing as part of the Instrument Rating skills test.
More information can be found on the UK CAA website under Guidance on specific routes to gaining a CBIR for ICAO license holders with 50 hours Pilot in Command in Instrument Flight Rules.