I am trying to find the website of a UK company called Salter Merit,
the company which owns the "Young Scientist" brand and makes products
such as Chemistry 100 and BBC Weather Activity Set.

Dear lawfulhippo,

I searched the database of the British Patent Office for the term
"Salter Merit" and found out that "Salter Merit Young Scientist" is
indeed a trademark registered and owned by Humbrol Ltd., better known
under one of their brands, Airfix, as manufacturers of plastic model
kits and similar toys. "Salter Merit" is not a company of its own at
all.

The official website of Humbrol Ltd. does, however, shows only their
Airfix model toys, while the products you mentioned can not be found
there. Nevertheless, Humbrol-Airfix is definitely the manufacturer of
these educational toys, as a quick additional search revealed.

You can find the Humbrol Ltd. website here:
http://www.airfix.com/

Please note that it is being redeveloped at the moment, the launch of
the new site is announced for the near future. This new website might
also include information about the company's educational toys.

Should you prefer to contact the company directly with questions you
might have, please use this address:

Humbrol Ltd.
Marfleet
Hull
HU9 5NE
England
Phone: (+44)(0)1482 701191 
Fax: (+44)(0)1482 712908
E-Mail: Corporate@airfix.com

Source:

The UK Patent Office: Trade Marks Database Enquiry
http://webdb4.patent.gov.uk/tm/text/

Search terms used:
"trademark registration" uk:
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22trademark+registration%22+uk&meta=
"Humbrol Ltd":
://www.google.de/search?q=%22Humbrol+Ltd%22&ie=UTF-8&oe=UTF-8&hl=de&btnG=Google-Suche&meta=
"Humbrol Ltd"  Marfleet:
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22Humbrol+Ltd%22++Marfleet&btnG=Google-Suche&meta=
"humbrol" "young scientist":
://www.google.de/search?q=%22humbrol%22+%22young+scientist%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=10&sa=N

Hope this helps!
Regards,
Scriptor

Thanks, this is great, especially for $2. I'll e-mail Humbrol Ltd. and
hopefully get the information I need.

Thanks again,
Lawful Hippo