Because up to the 16th of March, the UK's strategy was essentially to let the virus spread around the community so that long-term there'd be herd immunity similar to measles etc. There were no plans for wide-spread testing to track the virus, similar to say Korea or Germany.
Then on March 16th the Imperial report said something like 250k people (or was it 1m?) would die under the govs current plans as the NHS would be overwhelmed, they backtracked and since have wanted to test more, saying they wanted 10k+ tests a day by the end of March, and 100k a day by end of April. The problem is because they left it so late, they can't get the equipment for testing as every other country in the world has long before ramped up testing. You need both labs and the chemical reagents to run tests - the labs part should be easier as we have a lot of universities and private labs, but the chemical part is probably harder given we don't really manufacture much these days.