Science

Dear Mohit,
Specialize in a domain that you find interesting rather than what's in demand. Do not bother about jobs just yet. You should rather focus on gaining a quality education. However, answering your question, visit this link: https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-k-shortage-occupation-list.
In short, occupations in the STEM category are in demand so you may consider courses that relate to this categories-Science, Technology, Engineering and Mathematics. In order to apply to UK universities, you would require the following set of documents:
1. Class 10th and 12th mark sheets
2. Two recommendation letters from your teachers on school's letterhead duly signed and stamped.
3. An essay about yourself as a personal statement
4. Passport copy-Front and Last page
5. IELTS score of 6.0
Hope this helps.

Chemical is a core branch. However new it may be here, it has got some scope after the graduation. Chemical Engineers are in demand at mineral-basedindustries, petrochemical plants, synthetic fiber units, petroleum refining plants, chemical industries, refineries, pharmaceutical companies, paints manufacturers, fertilizer industry, textiles sector, plastics, and explosives.
Polymer Science is the field that is related with the science and technology which helps in analyzing and modifying the polymer materials. The topic covers the manufacture of polymers, its specifications as well as applications.While polymer is not a core and gonna-be-obsolete branch. Since,not too many colleges have this branch,so it's not so famous.
Hope this was helpful.