How do they know that 60% of cases in London are the "new" variant?

[Crunchymum]

Sorry if my % is wrong but hopefully people get the gist of my question?

Testing

There is constant surveillance, in UK and abroad, of which strains are present and what proportion of infections they represent

And notice they said it was also picked up in other areas of the country too, but as yet not as prevalent. If it is as highly infectious as they say then it will in time be the cause of exponential growth elsewhere unless action is taken with regard to schools.

Testing probably detected the virus but noted a mutation. I'm sure they said on the news a few days ago that Porton Down were studying it, so it must have a different structure or something that is detectable.
I'm not a scientist, though.