The first thing to be said about The Iron Lady is that Meryl Streep is remarkable as Margaret Thatcher.

As accustomed as we are to Streep's chameleon-like skills, she surpasses herself as the former British prime minister, her performance combining uncanny mimicry with emotional depth. It's a shockingly good piece of acting.

However, the film itself is another matter. Entertaining but limited, it is founded on a decision – to take the politics out of a study of one of the world's most controversial political leaders – which is at once bold and bloody absurd. For anyone who experienced the Thatcher years, it feels quite insulting.

Writer Abi Morgan and director Phyllida Lloyd seem keen to provide a very particular biopic, not exactly feminist, but focused on the experience of a woman's struggle to succeed in a man's world, and the personal cost of her single-minded career on her family life. Moreover, it is told through Thatcher's own, imagined perspective, her memories in old age and with the onset of dementia, during which time she also converses with her long-dead husband Denis (Jim Broadbent), the rock throughout most of her career.

Thus we're offered the view of the young Margaret (Alexandra Roach), the grocer's daughter inspired to enter politics by her father's belief in self-reliance and hard work, but confronted in parliament by chauvinism and class snobbery; her comical (although highly effective) attempt to transform her public persona, in her bid for the Tory leadership, and, in the present, the doting attention of her daughter Carol, lent poignancy by Margaret's absenteeism as a mother.

This approach, aided by Streep's warm-blooded performance, results in a portrait that is deliberately sympathetic, rather than analytical. It can't be a coincidence that the film leaps over the period between Thatcher entering parliament in 1959 and her position as education secretary in Edward Heath's floundering government 11 years later – a period that would have consolidated her economic thinking and a certain loss of innocence.

Likewise, while namechecking the major events of Thatcher's premiership – the miners' strike, the Brighton bombing, the Falklands, the poll tax riots – there is little insight into what turned an idealist into a tyrant, and no reflection whatsoever on Thatcher's legacy. And a great opportunity becomes a damp squib. David Cameron who, like Thatcher, seems determined to make the working classes and most vulnerable pay the greatest cost of recession, must be breathing a huge sigh of relief.

I'm pretty sure that the aim of Mother And Child is to celebrate the power of the maternal instinct and the bond between parent and child, but, if that's the case, writer/director Rodrigo Garcia has a strange way of going about it.

This singularly grey yarn concerns a woman (Annette Bening) turned into a harridan by her decision to give up her daughter for adoption, while the girl herself (Naomi Watts) has been so scarred by the abandonment that she's grown into a cold, manipulative loner. Another woman (Kerry Washington) is driven half-mad by her obsessive pursuit to adopt, her self-worth crushed by her inability to have a child of her own.

Although the performances, particularly that of an admirably chilly Watts, hold one's attention, this is too depressing by half, so much so that the contrived happy ending leaves one not only unmoved, but appalled.