It is possible to represent a witness. Funding would only be available under a CRM1&2. But you would need to justify attending on a witness. If a complex bit of law or a vulnerable client its probably fine. The simplest justification would be for a witness who is clearly at risk of self-incrimination.
The General Criminal Contract stipulates that a fee may be claimed for attending with a witness if there is a complicating factor and refers you to the Contract Guide. There is no such thing as a Contract Guide but the GCC is surely referring to any guidance issued by the LAA or other body.
There is no such thing as a complicating factor either. Its not in the LAA Criminal Manual or the Standard Criminal Contract or the Crime Contract Specification or even the Standard Terms. Hardly helpful.
But according to the Criminal Bills Assessment manual: 4. In the case of a witness who does not meet the definition of a volunteer, it must be demonstrated that they require advice regarding the risk of self-incrimination (Regulation 12(2) (i) of the Criminal Legal Aid (General) Regulations 2013).
It would therefore be sensible to check with the firm's Legal Aid supervisor.