For exact times in your particular locality you will need to do the following:
(1) Please visit http://astro.ukho.gov.uk/websurf/
(2) It would be necessary to agree to the conditions.
(3) Select ‘prayers’ option.
(4) Thereafter, enter postcode in the relevant section.
(5) Select the link of the postcode to go to the next screen.
(6) Thereafter select ‘Astronomical twilight / Sun’s depression 18° 00” There are also other options of 17.5° and 15°