This principle applies more generally than that in the UK. Someone I know very well got screwed following a formal grievance at work because of this. Due to bad advice, a deadline in the process passed without the right things happening, and the formal process technically ended. Although the person was actively seeking other advice over the following weeks, they continued to work, and then when things came to a head, basically the first thing the company lawyers cited was the continued work without further (to the company's knowledge) objection. For practical purposes, that was game over in this case.

Bottom line: If you aren't happy with a contract, either explicitly challenge it immediately or don't start to perform your actions under it until the matter has been resolved.