Where in the UK is area code 020 8342?

UK telephone area code 020 is London. A number beginning with 020 8452 is most likely a BT landline in Cricklewood, London; however, with number portability, any specific 020 number could be located anywhere in the greater London 020 zone (including the old "Inner London" zone), served by any local telephone company.

If this is a LONDON, UK, number then the correct format for the number is:

(020) 8452 xxxx

The area served by 8452 is Cricklewood and Dollis Hill.

These numbers were previously in the 0181-452-xxxx block, and before that in the 081-452-xxxx, block.

Way before that they were originally in the 01-452-xxxx block, pre 1990.