The regs say you can't make it WORSE (got them open in front of me), and that any alterations the existing stuff has to be right to start with (as in earth loop impedance, fault current etc). So you actually don't have to install a new consumer unit, but a new ring or radial has to be to standard (RCD etc). So provided you are simply extending what is already there, and not actually installing a new circuit, you are OK to just leave the consumer unit "as is". That said, wording can be tricky and although you would be within the law, it's not the best option as we all know.
As other folk have said, popping an RCD in there might be best, or even an RCBO. My consumer unit at home has one RCD before everything else, it's a pain but at least it's not going to kill anyone (hopefully!). But we also all know what will happen if you put an RCD in there straight off the meter tails, the whole lot will keep tripping and you'll get blamed for "doing something" because "it was ok before you touched it!".
Were I in your shoes, I would offer to put the sockets in, make it clear that although legal is not best, strongly advise an RCD be fitted at the very LEAST, and urge a consumer unit change sooner rather than later and perhaps provide some sort of documentation outlining this. Just my 2 pence, but then I'm only starting out as a spark the regs are very fresh in my mind as I've just had two weeks of exams and reading (though I do have 14 years offshore and military engineering behind me so do know a little about electrical lol!)