I get that you need to self isolate for 14 days if you're contacted by someone. And are only to test if you develop symptoms. But someone I know (and this isn't the first time I've heard this happen) got contacted by their friends to say they tested positive, so her and her partner straight away went to get a test (with not 1 symptom at all) and they both tested positive.
Now, if they had followed the government advice to just self isolate, then a) they wouldn't have known they'd had it and b) the people they were in contact with in the days between visiting their friends and getting the test wouldn't have been contacted. Therefore spreading it further. O personally think people should be allowed a test in this instance. They'd been in each others house, it was quite likely they'd possibly have it!