In the way you wrote the sentence, an Italian speaker will argue that you are in
London, as the other person came to London, so presumably you met him there.
With that meaning, the sentence is correct, usually it needs to be continued,
as in:
Ho conosciuto un tale, venuto a Londra e in tre giorni abbiamo visitato insieme
tutti i musei che avevo fino ad allora ritenuto noiosissimi;
la sera andavamo nei pub, ci siamo davvero divertiti.