This was explained by (iirc) Jenny Harries. By the time we were aware of the outbreak being apparently imported in Europe, it was already actually so widespread here that they could never employ enough contact tracers to deal with it. There was vertically zero testing capacity as we simply didn't have similar medical industry set up to (for eg) Germany. Therefore, the only option was to lockdown, build testing and tracing capacity, then begin test and trace once disease levels were minimised and lockdown could be lifted.
No, I don't accept this. Read the SAGE minutes.
They discussed contact tracing on 11 February. They didn't discuss how to build an effective contact tracing system or how to increase capacity. They asked SPI-M and PHE to work out when they could stop doing it.
We had 8 confirmed cases on 11 February.
www.gov.uk/government/publications/sage-minutes-coronavirus-covid-19-response-11-february-2020
PHE reported back on 18 February and said they could cope with tracing the contacts of five cases a week, based on an estimated 160 contacts per case (!) Again, there was no discussion of how to scale up the system or who else might be able to help besides PHE (e.g. local government - www.local.gov.uk/lga-statement-coronavirus-contact-tracing-strategy). There was no discussion of how to work out which of the ~160 contacts were the most important to trace. Once again it was all just 'when can we stop doing it?'
We had 9 confirmed cases on 18 February.
www.gov.uk/government/publications/sage-minutes-coronavirus-covid-19-response-18-february-2020
It's true that as time went on we also didn't have enough testing capacity but this wasn't just bad luck. Once again it was mismanagement and a lack of will. IIRC the initial problems were largely due to wanting PHE to run it all and ignoring testing capacity that was being offered elsewhere.
Fairly early on in this pandemic Jenny Harries was asked at a press conference why we were not following WHO advice (which was test, track, isolate). She said WHO advice was for low and middle income countries and we didn't need to follow it because we had a well developed public health system. I'll see if I can find the clip tomorrow.
And so here we are a few months later with a shockingly high death rate, having to play a massive game of catch up on testing and contact tracing while being put to shame by rather a lot of low and middle income countries which have handled this far better than we have.