Whilst working with various sized accountants and lawyers I have noticed the difference in how practitioners view the provision of registered office.
Some regard it as a "freebie" to their clients whilst others charge premium. My take on this is why not charge?? Even if you charge £100 and have 100 companies that's £10,000 annual REPEATABLE income And if you do want to give it to your client as a freebie then let them know that it is a freebie and what the market rate is! You may find it's a good marketing ploy!