Negative attitudes and hostility towards gifted students is a real, widespread and longstanding problem.
In the US the Marland report (1972) concluded that Identification of gifted students was hampered not only by testing costs, but by both apathy and hostility among teachers, administrators, guidance counselors and psychologists
In Australia the phenomenon is so common it has a name: cutting down the tall poppies
Geake and Gross studied Teachers’ Negative Affect Toward Academically Gifted Students
Hoogeven noted a case where "Other students do not accept him [an accelerated student], partly because they are jealous" and notes in a second study that "we found that the general attitude of secondary school teachers concerning accelerated students is not accurate and rather negative. Cornell (1990) also mentioned prejudicial attitudes in the classroom or school as a possible cause for unpopularity"
And if you want an example of batshit crazy hatred try this.
So the OP's original observation that some (not all) people hate gifted children is entirely correct.
As for attitudes in the UK:
the UK is the only country where the national association for gifted children has gone as far as changing its name specifically to avoid including the word gifted, and changed all their documentation to avoid mentioning the G word at all.
writing about the G&T scheme the Guardian notes " hostility towards a scheme some teachers regard as divisive" and that many G&T educators can cite cases where teachers have included less able students on the programme "to deliberately disrupt the course"
Another interesting persepctive here: "When Simone, an ECHA Specialist in Gifted Education, moved to the UK from her native Netherlands she was shocked by how giftedness is perceived here. Simone’s first surprise was her discovery that in Britain hardly anyone uses the word ‘gifted’. When, as part of her research, Simone began asking parents about giftedness, their reaction was actively hostile. Most Brits, Simone discovered, perceive giftedness as elitist and as conferring even more benefits on already overly-advantaged white, middle-class children ... Simone found that it was only by avoiding any mention of giftedness that she could reach the people she was trying to help."
So, why? I suggest:
Tall poppy syndrome
Envy / Jealousy : other children / classmates wish they were that smart. Parents /teachers wish their own children were.
Insecurity : teachers feel threatened by children whose competence in a subject approaches or exceeds their own, or who may point out mistakes or inconsistencies.
Lack of respect for diversity. schools talk a good talk about respect for ethnic, cultural, religious and gender diversity, but when it comes to intellectual diversity they'd rather pretend we're all the same. gifteds kids are sometimes ostracised by chronological peers with whom they have very little in common.
Resentment: gifted children are harder work. they already know what's on the lesson plan for today and the curriculum for most of the year. differentiation for the gifted puts a burden of extra work on the teacher. Most teachers know they should be doing it, but half of them cant be arsed. Gifted kids are inconvenient.
Anti-elitism, misplaced egalitarianism, and confusing equal opportunities with equal outcomes.