They won't announce a lockdown in advance if its for a certain area of the UK instead of the whole country because some people would leave that area and then risk creating another hotspot.
Someone tested positive in London but went to his second home in Polzeath, Cornwall to self isolate. He went into the little shops to get supplies and they then had to then close for a deep clean. This was a couple of weeks ago but it just demonstrates how selfish people are. And some people, if given pre warning will move around the country, thinking only of themselves.