Is it legal to start an online scratchcard (not lottery) website in
the UK with a view to profitable gain?

<Under UK law, if a competition does not involve skill, it may be
considered a lottery. To be exempt from the controls on lotteries, the
law requires that success in competitions must depend to a
"substantial degree" on the exercise of skill.
 
There is no statutory definition of what constitutes "skill". We would
advise anyone who plans to promote such a competition to seek legal
advice.

Prize Competitions 


Definition of a prize competition
Prize competitions are where success depends to a substantial degree
on the exercise of skill. Such competitions usually involve the
answering of questions, solving of puzzles, tie-breakers etc.

The relevant legislation 
The law on prize competitions is contained in section 14 of the
Lotteries and Amusements Act 1976. The Act is available from the
Stationery Office.

Source: Department for culture, media and sport.
http://www.culture.gov.uk/gambling_and_racing/fact_sheets/fact_prize_competitions.htm>