That R0 was falling below one prior to 'lockdown' has been acknowledged by Chris Whitty in evidence to a parliamentary committee (see below, 1:14:00 to 1:15:30)
Now, firstly, kudos for including the link to Whitty's appearance at the committee. Most people just state that so-and-so said this, without any referencing, so the fact that you didn't do that demonstrates to me that you're open to intellectual rigour and challenge.
With that in mind, it's important to be clear what Chris Whitty actually said in his evidence and the context in which he said it. He was asked whether the government had ignored scientific advice and locked down one week too late in March. His answer was:
"There was a package of things that were strongly recommended on the 16th and those happened there. There was subsequently clear advice to close schools, which previously had not been advised, and that happened subsequently
and then, after that, it was clear there was further acceleration, or at least less clarity that we were going to be able to get R below one...so advice was to go further than that...
"...and to be clear, multiple steps were taken along the way and if you look at the R...quite a lot of the change
that led to the R going below one occurred well before, or to some extent before, the 23rd when the full lockdown started...
"...and
I'm very much in favour of the fact that the lockdown happened but, actually,
some people argue that R crossed one even before that point in time".
So, firstly, it's important to note that he was providing answers as to why the government (of which he is a senior official) had not locked down too late. He was not providing evidence as to whether they had locked down too soon or whether lockdown wasn't necessary at all, as he wasn't asked those questions.
Looking at his specific answer, he firstly outlines that a range of measures were introduced before full lockdown on 23rd March. He then states that the evidence suggested that, despite those measures, infection rates were still accelerating (or at least not reducing), so further measures were then required.
He does acknowledge, however, that some of those original measures will have assisted with the R eventually falling below one. Crucially, though, he doesn't state that they were sufficient by themselves, nor does he state that R had fallen below one before 23rd March (in fact, it's clear that he believes that is not the case).
Finally, he reiterates his belief (several months after the fact) that lockdown was necessary. He acknowledges that "some people" argue that R had fallen below one before 23rd March, but the totality of his answer, and the context within which it was given, make it clear that Chris Whitty is not one of those people.