Define Mass?
2.5% of the population is of Indian heritage circa August 2020 according to UK GOV stats.. That would mean we would have to allow around 30 million Indians to arrive here before they became equal in population to white British. You are also assuming that Indians here do not assimilate into British culture. As British culture was transferred to the Indian sub continent during colonialism they are more likely to have afternoon tea than the British.
It is reasonable to not want the nation to become subverted to another culture, or for as you describe native British/English to become a minority, the figures suggest this is not happening, nor is it likely to happen.
Education sometimes causes surprises.
History is important, it should in theory stop us from making the same mistakes all over again.
We do, how else do we know about Nelson, Wellington, etc etc, but culture is not constant, it is evolves. The culture of 2021 is very different to the culture of 1971 which was very different to 1671 and so on.
You are describing communities that are insular, due the nature of Empire and colonialism the British have always been a nation that is outwards looking and welcoming to foreign cultures, think how going for a curry is now acceptable as a cultural norm.
I am in favour of controlled immigration, what i am not in favour of is the skewing of the debate so that we hear words like "mass" "swamped" "illegal" etc as it does sour the debate and the myths become accepted, one being the idiotic "great replacement theory." It also harms the immigration we do need as a nation, we have a negative borth rate and an aging population, if all immigration is frowned upon, basically as a nation in time we will die off and all those people claiming lineages back to William the Conqureors chief cook will find we have no nation left.