I don't see how the tracing will make any difference to people who need shielding.
It's supposed to be starting tomorrow, but that seems doubtful. I read somewhere it's going to be a manual tracing service.
Anybody who thinks he/she is infected is supposed to inform somebody (by phone or email or text - not sure). That person will then be contacted by a tracer and sent a testing kit in the post. The results are likely to take at least 48 hours. If the test is positive, the person will be contacted and asked to list anybody with whom he/she had been in close contact, who will then be asked to self-isolate.
Two problems spring to mind. Firstly, it depends on the initial person knowing the name and contact details of everybody he/she has been in contact with. During the 48 hour wait for test results, any number of contacts could be wandering around infecting others, unknowingly. One of those could very well be a highly vulnerable person.
Personally, I wouldn't be prepared to take the risk. I would want to keep any contact with others to an absolute minimum, unless I could be totally sure that any other person had been self-isolating. I am extremely relieved I'm not in the position of being forced to work in an environment where self-distancing is not possible.