Looking at COVID models, the hospital admission peak is around the 10th of January UK wide. Cases have already started to level in London. Lockdown has been implemented too late to make any significant difference to that. Not the most reliable source, but confirmed by other sources which are unfortunately behind a paywall. https://www.dailymail.co.uk/news/article-9115357/amp/Coronav...
This is a common claim and I have seen it made about various periods of lockdown in various countries. "Look, rate of increase started to slow within a week of lockdown, but it should take two weeks to see the effect, so it can't be that lockdown has made any difference." There is usually far too much noise in the figures to make such a claim about small differences in time scale. Also if you were expecting a sudden drop after two weeks then you were bound to be disappointed. The amount of time it takes for an infection to become a recorded case of Covid will be random according to some distribution (possibly with a mean of one or two weeks) so the transition will be smooth.