I am afraid your teacher lied to you, Britain is still over 90% white and whilst atheism does seem to be prevalant, officially over 70% of the population is christian. Whilst Britain is definately predominantly a tolerant nation and can be proud of this fact, embracing many different races, religions and cultures, your teacher has told you complete lies. Britian is certainly not 96% pro-EU, it has widespread support for euroskeptic parties such as UKIP and many recent polls show that the vast majority of the public are skeptical and even anti-European-Union, not too mention the incumbent conservative party has over 100 members of parliament who are outwardly anti-EU.
Lastly do 97% of Brits support multiculturalism? firstly statistics cannot be provided for things like beliefs and support, who knows what percentage of brits support multi-culturalism, but definately the real percentage is nowhere near 97% and I would hazard a guess that it is more like 50% as many different groups and parties vehemently oppose multiculturalism and many parts of the country have yet to even experience multiculturalism.
You must ask yourself why your teacher would tell you something like this? Do not let other people's bias sway your opinions on the world, make your own opinions using facts, dont make up facts to strengthen your opinion.