You may wish to work with other professionals in connection with an engagement for one of their clients (an "ultimate client"). This is rare, but when it does happen it is frequently following an approach by another law firm or an offshore corporate services provider.
When this happens, you need to take care as to how you will be engaged by the other professional. In particular, are you working as a consultant, under their supervision and on their account (this is only an option where the other professional is itself a law firm), or, more likely, are you working as an independent private practice lawyer and on Keystone's account? The way you engage determines the style of the Engagement Letter you need and who you set up as the client.
Law firms seeking to instruct you routinely misunderstand this. There are two acceptable ways to work with other firms: acting as co-counsel, or acting as a labour provider. For guidance on co-counselling see Acting as co-counsel with UK and foreign law firms. The other option, which is rare and not recommended, but possible for some niche areas of law, is working on a labour-only basis. Here, the host firm treats you like any other member of the firm, or a locum, albeit it through Keystone. All the work product received by the client is created by the firm engaging you, which, accordingly, has all the risk and most of the reward. On this basis, the host firm is your client and you either have no contact with the ultimate client, or you do, but only in the name of the host firm. The ultimate client is not aware of Keystone. Keystone and you therefore have no liability, save for fraud. This can have many pitfalls, accordingly, you should contact the Director of Operations and Compliance before agreeing such an engagement.
You may be asked, even by big firms, to act for them as part of their service delivery and not on a labour only basis. You can act for that firm only where they are consuming your advice. If they wish to rewrite your advice or otherwise pass it on as theirs, then that is fine: their client will not know of Keystone and Keystone will have no liability to their client. This is permissible, but again can have many pitfalls, accordingly, you should contact the Director of Operations and Compliance before agreeing such an engagement. You must not act for another firm and then have any contact with the ultimate client or for the ultimate client to become aware of our involvement. This is because we must have a binding contract with the party consuming our advice. You put yourself and the firm at risk if you allow a law firm to be your client and the ultimate client is aware of your existence. You will have breached the SRA Code and may end up with personal liability.